Why aircraft pilots are certified by hours they have flied instead of the distance?

Airplanes don't have odometers like a car does. There is no way to measure distance traveled because the airplane does not fly at a constant speed.

It is not only the hours flown that certifies pilots, it is also passing the knowledge and skills examinations in written, aural and performance. The hours like 20 hours of dual and 20 hours of solo, mean nothing if the training curriculum was not followed that put meaning to those minimum hours.

It is not a practical way to measure skill. A pilot who has spent 50 hours practising IFR approaches to minimums will have greater skills than some one who has flown 1000 miles straight and level in perfect conditions.

It is not what distance you have flown which counts, it is the skills you have gained in the time you have been flying.

And how exactly would you measure distance for a training flight where the pilot takes off, spend a couple of hours doing turns around a point, steep turns, slow flight and stalls, then returns to the same airport and spends time in the pattern doing touch and goes?

It's a lot easier to measure the time flown than the distance travelled.

It goes back to the very earliest days of flying, where the hours really did define exactly how much experience you have, because hours defined how many difficulties you had encountered and managed to survive. Since that is still the situation today, no-one has ever thought there was any point in changing.

because for example helicopters, spend hours hovering which is their job.

besides, you can convert the flight hours into distance travelled by approximation and simply by looking into the logbook (it includes point of departure and point of landing, sometimes the route, too)

Why? Because planes travel at different speeds and it is time spent airborne that is the preferred measure of overall experience.

They say there is no such thing as a stupid question. "Nuff said.