A car requires only 1 single driver. Why do airplanes require 3 pilots given that each pilot does not come cheap to the airline? Furthermore, a plane can go on auto-pilot while a car cannot. Could there be more room for efficiency by reducing the number of pilots needed?
Actually, for all but very long haul flights, airline aircraft only require two pilots, a Captain and a First Officer. For those long haul flights that exceed the duty time in a pilot seat, a relief pilot is also carried.
Older aircraft often also had a flight engineer. Technically speaking the f.e. wasn't usually required to be a pilot, but he was a very necessary crew member. You could say that airlines became more efficient crew-wise when they moved from 3-man cockpit crews to 2-man cockpit crews. For example, the 747-100/200 series uses 3-man crews, but the 747-400 uses 2-man crews. The change was made possible by using automation and instrumentation that wasn't available when the first 747 models came out. Indirectly the increase in engine reliability also helped. If you lost an engine in an 3-man crew 747, the typical manner of handling the loss was to have the first officer fly the airplane while the captain and the f.e. worked the problem.
To illustrate the increase in automation, the 747-100/200 series had, typically, seven or nine fuel tanks, and there was a fuel burn schedule that had to be followed to keep the structural stresses and balances between different levels of the tanks within limits. The f.e. took care of that, doing it manually by throwing switches on his panel. The 747-400 and follow on models do that automatically by computer.
Basically the number in the cockpit crew is determined by safety. There's the basic consideration that if you had only one pilot, and that pilot died or otherwise was disabled, that might well be catastrophic. Beyond that, though, there's the matter of what all has to be done and for how long. Pilots make mistakes, and the longer they've on duty, the more mistakes they make. And if an emergency occurs, the work load can get very high.
You can't reasonably compare an aircraft to a car for at least two reasons. First, a car is moving in only two dimensions, an aircraft in three. Second, a car can stop moving, an aircraft can't; it has to keep moving. Let's say your car loses power. Usually that won't result in anything more than inconvenience. If your aircraft loses power, you've got a much more serious problem. Most airline aircraft have at least two engines. Losing one isn't going cause a crash in most two engine aircraft, losing all power will.
Most of the commercial airlines now require only two pilots- a captain and a first officer. It is in a large part, regulatory (GA and combat aircraft have only a single pilot). It is perfectly possible to fly a transport aircraft with a single pilot.
In fact, there have been calls for eliminating the second pilot. There are some points though.
The roles of the captain and first officer are complementary, not identical; though it is perfectly possible to have only a single pilot, having two helps.
One important reason is safety/redundancy- If something happens to a pilot en route, the other one can take over and land safely. this safety by redundancy extends to all aircraft systems.
In long haul flights, two (or more, usually) pilots are required (both practically and by regulation).
Another point is that autolanding is not possible in all the airports. it needs the necessary ILS.
One problem with comparing aircraft with cars (or other surface vehicles, like trains) is that the effects of losing control and power in these cases is completely different. A car can simply go hit something and stop- that is not an option with an aircraft; it'll fall. Trains usually have a dead man's switch that'll apply emergency brakes if the pilot's presence is not felt.
In a land vehicle most of the time if something goes wrong, either mechanically with the vehicle or medically with the driver you can just stop. Stopping may be inconviniant but it's unlikely to be deadly.
An airplane on the other hand is a complex machine (and the bigger they get generally the more complex they get) and if something goes wrong requires careful management to get it safely back on the ground.
Autopilots can be helpful in some phases of flight but they still require manual input and landing on autopilot requires ground infrastructure that not every airport has.
For light planes with small numbers of passengers (or no passengers at all) the risk of having a single pilot is deemed tolerable. On the other hand having an airliner crash because one person was incapacitated is not considered acceptable.
So the flight crew on larger planes is one larger than is strictly needed to fly, navigate and land the plane. Older planes had three flight crew (two pilots and a flight engineer), more recent airliners have added more automation and got rid of the flight engineer.
Airlines sometimes elect to fly planes with more pilots than is actually needed.
On reason for that is training.
Any time spent in the cockpit is increasing the pilot hours rating, even if only seated in the jump seat, without access to controls.
On a side note, adding an other pilot might also be helpful in case of an emergency; this was considered as a contributing factor in the relative success of the UA232 crash for exemple.