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If the airline has personnel to maintain the aircraft, wouldn't it be more cost effective to have them inspect the aircraft before flight rather than have the captain or first officer do it?
Is it because whoever will fly the aircraft will probably do a better job of inspecting it because they are putting their own lives at risk?
Because it's the Captain's plane, for all intents and purposes. This is exactly the same for many other industries - in particularly shipping (as in boats) and haulage (as in trucks).
Similar to planes, ships and boats have captains and they are all fundamentally and legally responsible for everything that happens on that mission. And, likewise, a truck driver is expected to maintain responsibility for their truck - indeed, over here in the UK this is a legal requirement and truck drivers have to do a mandatory walk-around check.
It's also the same for car drivers - you, as a driver, are fundamentally responsible for your car and its safety.
Focussing on aircraft, the pilots probably have the single most holistic view of the aircraft, its mission and the requirements. In addition, the pilot is always going to be there - whether that's Heathrow at 2pm on a Tuesday, or a deserted dirt strip in the outback at 10pm.
The better question might be - what good reason would there be to skip a walk around in one of the most safety-critical industries? At best, for some airports in some circumstances, you can argue somebody else might be able to conduct the walkaround - but it's not guaranteed and it introduces another potential point of failure or miscommunication.
As @Gerry has posted, in the US, FAA regulations place the responsibility of maintaining the aircraft on the owner/operator. But, the aircraft’s airworthiness is the responsibility of the Pilot in Command. That is the regulatory reason.
The practical reason is one of redundancy and common sense. An aircraft needs to be inspected before every flight. Although, a mechanic will not always be available before every flight to do that, the PIC will.
Also, both the mechanic and the pilot are human. It makes more sense from a safety point of view to have as many different eyes inspect the aircraft for deficiencies. What one person may overlook, another person may catch. The better question is, why in a two or more pilot system, why is each pilot not required to do a walk-around inspection.
When I fly small GA aircraft with another pilot, one of us does a full and detailed preflight inspection. The other pilot does at least an abbreviated preflight inspection on the outside of the aircraft. They then participate in the preflight inspection and run-up in the interior. At the bare minimum, the other pilot has to visually inspect the fuel and oil quantity, flight surfaces, air/cowl inlets, and that all caps, doors, hatches, tie-downs, and chicks are secure. Once inside, they have to check that all control surfaces are free, clear, and correct, and that the brakes work.
The pilots are the only people who have all three of these attributes:
The training and experience to know what is safe.
The legal authority to stop the flight.
A deep personal stake in the outcome of the flight.
That is the safety trifecta. A mechanic has the training and experience to judge the safety of the plane mechanically, and the authority to stop a plane from flying, but does not have that deep personal stake in the outcome of the flight. I'm not saying that the mechanic doesn't care, but it's human nature to care more when you could die from your mistakes. Although the pilot's training does not include the depth of mechanical knowledge that the mechanic has, the pilot is trained to spot the things that can be spotted visually that indicate an airplane is unsafe to fly. Not all mechanical mistakes can be spotted by the pilot, but many can. The pilot is looking for things out of place, missing fasteners, leaking fluid, deformation, and anything else that can indicate an aircraft that is not safe.
As an aside, this safety trifecta is why passenger planes piloted remotely or by automation are a terrible idea. The pilots are removed from the equation, and now the people who are deciding whether the plane flies or not don't stand to die if they get it wrong.
I'm not really satisfied by the answers "they walk around because they're required to do so".
Besides the fact that others might have a lower stake in the health of the plane, it's worth noting that over the decades, the culture of the aircraft industry has been forcefully changed, due to crashes happening for dumb reasons.
In an episode ("Check Yourself") of Hidden Brain, there was a particular focus on a crash at a flight show back in the 1940s or 1950s, where they were introducing the B-52 bomber (in service since 1952). Since it was at a flight show, obviously these were top pilots who were doing the demonstration, but anyway, they pitched up too steeply from takeoff and fell out of the sky.
Some people survived the ensuing fire thankfully. It turned out that some sort of flap was locked in place, which was meant to act as a brake on the ground; and had the effect of increasing the pitch. This would typically be unlocked from the outside, but in this new model, it was done on the inside. It seemed to be that unlocking it skipped the mind of everyone and if they had realised in time upon takeoff, they might not have been able to lean over and turn it off anyway.
The naïve solution of "just be more careful" wasn't feasible − these were accomplished pilots; and pilot training could only take so long. I encourage you to listen to the episode, but to summarise, the main thing to come of this investigation was the decision to start using checklists of 6-10 checks − the top few dumb reasons that might cause crashes.
Over the years there were other scandals too, eg Korean Air, which was largely blamed as a cultural issue. Korea has a culture built heavily around respect for different classes − your boss, your elders. This culture is seemingly more extreme than what exists amongst its East-Asian neighbours; and it's built into the language as different forms of address. If they're asking for something or trying to make a point, they beat around the bush because they're terrified of being too blunt.
As the remedy for all the crashes, the pilots were encouraged into the notion that it's OK to speak up if you think something's wrong; and more drastically, they were also told that from now on, they had to speak English, like all the other pilots and airports were typically doing.
English doesn't have the different forms of "you" like so many other languages do; and similarly, trying to express all this indirect desire in English is avoided because it sounds weird − there's an example of a Korean pilot repeatedly expressing that basically he was a fan of weather reports, in regards to his captain blaming the weather reports for the fact that he was unexpectedly flying through clouds, in the wrong place, about to crash into a mountain.
In another industry, it might indeed seem normal to allocate someone else to walk around the plane. If someone else said afterwards though, that they too were going to inspect it, it would probably be perceived as a sleight − "I already checked it. Are you saying that I'm blind?"
If it were as simple as saying "the pilot has the most at stake, so she has to do it", then we could point to seatbelt laws − in eg NSW, Australia, it's the driver who gets punished, if some of the passengers aren't wearing seatbelts.
In this aircraft scenario, the pilot has internalised the fact that he has the most responsibility. He wants to check the aircraft, and it's not going to offend anyone else who was already expected to notice anything.
It is part a pilots training to do this. He is the Captain and he/she is responsible for the passengers lives. Tradition and common sense. Especially for small aircraft where no other qualified people are there.
Just to add another argument to the above questions: having a pilot do a walkaround instead of a maintenance worker includes another point of view. Maintenance crew usually work in teams and assumptions can easily be made.
Imagine a hatch that's not sealed. A maintenance worker might assume some other worker will close it or is still working on it.
A pilot seeing the hatch open will not assume this and see it for what it is; something out of order.
