As most crashes are on takeoff and landing, and fire is the real killer, most often. Why not have breakaway wings? That's where the gas is. Let's assume a plane has to make a hard landing..upon touchdown/contact jettison the wings. They serve no purpose at that point and they definitely kill people. Breakaway parts/modules are employed in many arenas, so why not wings?
"It's not complicated."
As with most things in aerospace engineering, it is actually a lot more complicated than someone not familiar with aircraft design might think. There are a number of reasons why this isn't done:
Wings have to be really strong. They literally bear (almost) the entire weight of the aircraft in normal 1 G flight and multiples of the weight of the aircraft when maneuvering. For example, in a 60 degree bank while maintaining altitude, the load on the wings is twice the weight of the aircraft. Of course, airliners don't normally do 60 degree banks, but they're designed to be able to withstand loads much higher than what they would normally experience on a routine flight.
In order to keep the wings from separating from the aircraft under such loads, they're designed with a single, very strong structure called a wing spar that runs most or all of the wing span. This is not one spar per wing, but rather a single spar that runs from one wingtip, through the fuselage, to the other wingtip. This spar is responsible for holding up the weight of the fuselage during flight and the weight of the wings (and, depending on where the gear is located, possibly also the fuselage) while on the ground. Using a single structure running the entire span for the spar is the safest and most efficient way to bear the needed loads, as it removes any connections that would otherwise be possible points of failure.
By design, the spar is really hard to break. Separating the wings from the aircraft would require breaking the spar. This would require enormous amounts of force and/or heat. The systems to supply that force and/or heat would be heavy and also, by design, rather dangerous. Neither of these is a good thing for an airplane.
Main spar of a de Havilland DH.60 Moth Source: Wikipedia
It turns out that the wings (especially of an airliner) are actually more useful than you might think during the landing roll.
During a landing, the primary methods of slowing down are wheel brakes, engine reverse thrust, and aerodynamic drag. All three of these, at least to some extent, depend on the wings being attached to the aircraft. The engines, flaps, and spoilers are all usually mounted to the wings. The wing flaps and spoilers aid in providing large amounts of aerodynamic drag during the landing roll. The engines also require fuel flow and, as you've noted, that usually comes primarily from the wing. The wheel brakes might have some effect without the wings, but it would be greatly diminished. Braking action requires weight on the wheels. A lot of this weight comes from the weight of the wing structures and more comes from the downward aerodynamic forces provided by the spoilers.
During the landing roll, the wings are still aiding quite a bit in controlling the attitude of the aircraft. In particular, the ailerons are still effective during the high-speed parts of the landing roll and are used during that time to keep the aircraft level in the roll axis. Additionally, a tall and very long tricycle, by itself, isn't a terribly stable vehicle. Especially at high speeds, it would be more prone to tipping over without the wings attached.
Take a look at the main landing gear on these aircraft:
Boeing 777 Source: Wikipedia
Boeing 757 Source: Wikipedia
Boeing 737 Source: Wikipedia
Airbus A330 Source: Wikipedia
As you will notice in all of these pictures, the main landing gear are attached to the wings.
It's very common in aircraft design for the main landing gear to be attached to the wings. This makes sense as the wing root is structurally a good place from which to support the weight of the aircraft and also happens to provide a nice, wide wheel base to keep the aircraft more stable on the ground.
If the wing separates from the aircraft at touchdown, the main landing gear goes with the wings. As you can imagine, this creates lots of problems for a landing roll. In addition to the loss of wheel brakes, you are now skidding down the runway on the bottom of the fuselage instead of rolling down it on wheels. Hopefully I don't need to go into the reasons why this is not considered to be desirable.
Risk of Failure
Any system can fail. If one of these systems accidentally activated (either by accidental command from the flight deck or by failure of the system,) losing one's wings while in flight is a rather bad outcome. Given that flights needing wings are extremely common and flights crash landing in such a way that jettisoning the fuel tanks would be desirable are extremely uncommon, having such a system around is probably more likely to kill someone than save someone.
While one might think that jet fuel is easy to burn, it actually turns out that it isn't. In fact, it's rather difficult to ignite it. This is intentional in order to minimize the chance of the fuel igniting during an accident. Even in the rather violent crash of Asiana 214 and the large post-crash fire, the fuel never ignited (see first paragraph of page 37.) So, the risk being mitigated here might not actually be as large as one might think.
Note, however, that avgas (used commonly in light, piston-engine aircraft) is quite different from jet fuel. Avgas is similar to normal car gasoline and will burn rather easily.
As previously mentioned, a system capable of quickly breaking the main wing spar would be heavy. It would be expensive to design to be safe. It would be expensive to build it. It would also be expensive to carry all of that extra weight around on millions upon millions of flights per year. All of this cost and added safety risk is not deemed to be worthwhile to provide a system that is rather unlikely to ever be useful.
I can't think of an accident scenario where this might help. If you're hitting the ground hard enough to crash but have survivors, you're looking at an accident like TK 1951, where not only was the wreckage all in pretty much the same area, but it didn't even catch on fire. Aviation fuel is not as volatile as movies make it seem.
Maybe you are thinking of something like AF 358 which overran the end of the runway and burst into flames? Despite that fire everyone survived, but what if they realised it was going to overrun and jettisoned the wings whilst still on the runway?
Well the biggest problem to me is how to slow the aircraft down without wings. At higher speeds, the wheel brakes are ineffective without the use of ground spoilers. Without the reverse thrust from the engines you are guaranteeing that the aircraft will be going faster when it leaves the runway, leading to a much higher chance of casualties.
No piece of engineering is perfect either - what happens when it fails and the wings accidentally detach when everything is going well? What if they are flying in severe turbulence and the sensors detect high G forces and confuses that for a hard landing? Or if it a manual jettison, what if the pilots accidentally pull it, or a suicidal pilot does it while the other is looking the other way?
There are many other problems too, such as how some aircraft are designed with the gear support in the wings, that some have large fuel tanks in the fuselage, and the extra weight and cost of getting such a system certified. Ultimately this suggestion falls into the same bucket as every other drastic safety idea - in trying to fix a problem that barely exists, it creates a lot more in its place.
Just wanted to add that if wings were to be 'disengaged' when needed/emergency-case, the aeroplane-fuselage or a 'metal Tube' now will fall faster than a human would free fall, there will be nothing to slow it down (assuming pilot/autoPilot can still control wing-control-surfaces or whatever its called).
If the aeroplane is very close to ground or on ground and moving, discarding the wings now means throwing them away with undercarriage, the wings would most likely tumble and continue to move with the plane and may be even into it.
But if the aeroplane is on ground and not moving and then the wings are disconnected, means the Plane still has wings next to it, but now the fuselage is below it. The fuselage could roll and make it difficult to evacuate. Also, lot of aviation fuel (30% or more) is in Central tank + ACT. i.e. in fuselage.
The early 310s with "tuna tanks" were designed with exactly this scenario in mind. The idea was that the tanks would keep the fuel as far from the cabin as possible and shear off in a crash. Cessna abandoned this idea some years later when they incorporated aux tanks into the wings (310L) and then later into the nacelles (310N).
I do not know if data exists to quantify the effectiveness of this design feature.