The main reason that parachutes are not used is that there are very, very few aircraft accidents that occur with enough time to actually use one. In fact, I'm not sure that there have been any. Below are a couple examples that you may originally think that parachutes could have been useful on because they initiated from a high altitude.
Air France 447:
The short version is that they stalled the airplane at cruising altitude and held it in the stall until it crashed into the ocean.
Let's consider that the amount of time from the peak altitude of the aircraft until impact with water was only 3 minutes and 21 seconds. Let's be very generous and say that everybody on board immediately knew that the aircraft was going to crash and there was nothing that the pilots could do.
This gives the panicking passengers just over three minutes to get their parachutes out of storage, properly secure themselves into the harness (which, trust me, isn't as easy as it sounds even if you know what you are doing and are in the proper frame of mind), all while in the confined space of an aircraft seat with everyone else around them doing the same thing. After that, we somehow need to open the doors and get people to orderly line up and exit the airplane without freaking out and getting scared.
Seriously, 99% of the people won't even get their parachute on (correctly) in that amount of time, much less the far less time that they would really have before they knew about the crash.
The reality of the matter is that the pilots are doing everything possible in an emergency situation to not crash in the first place, and if they are successful (which they usually are) then having everybody jump would have caused many more problems than having the passengers stay put in their seats with their seatbelts on. In this particular crash, the pilots didn't even realize that the crash was certain until four seconds before impact when one of the pilots verbally stated "we're going to crash". Up until that point they were focused on recovering the aircraft and probably would never even have given the evacuation order had it even been available.
Gol Transportes Aéreos Flight 1907
This was the mid air collision over Brazil. At least in this case, it became certain pretty fast that the aircraft was out of control and was going to crash.
A small quote from the [accident report] describes what happened immediately after the collision:
Immediately after the collision, PR-GTD started a fast descending
spiral, similar to the maneuver known as spin, which by no means
could be recovered or controlled by the crew. During the vertiginous
dive, the aircraft was submitted to extreme aerodynamic forces,
around all the axes, with positive and negative accelerations, well
above the maximum resistance limits of the operational envelope. As a
result, there was an in-flight break-up of the aircraft in several
pieces of different sizes, which hit the ground.
The increased G forces on the airplane were very likely to the point that people couldn't stand, or at the very least would have a much harder time doing so. Trying to put a parachute on in these circumstances would be even harder than in the previous example. Total time from the mid air until impact with terrain: Estimated 1 minute 5 seconds.
Each of these scenarios assume that even if there were parachutes on board and people were able to don them correctly in time, that they would be able to use them to survive. Here are a few additional factors that would come into play in the unlikely event that they were able to even get to this point:
- Modern aircraft doors can not be opened in flight.
- If this happened at high altitudes, everybody would need oxygen as well or they would pass out.
- The passengers that actually make it out of the airplane do not know how to fall in a stable position, and the parachute is very likely to become tangled while opening as they are tumbling through the air.
- The passengers would need to actually deploy the parachute manually while most probably in a panicked state of mind.
- There would be many injuries during landing.
- Once they did land, they would have no survival gear. This is particularly a problem over the open ocean or in the jungle (where these flights have been).
Considering that even in most accidents the aircraft is still landed in a somewhat stable manner and most people survive, having a couple hundred people bailing out of an aircraft would most likely cause more harm than good, even if they could solve all of the technical issues. If the decision is made to evacuate and then the pilots get the aircraft back under control, it would be even worse!