Question

Airports at high elevations have longer runways for takeoffs and landings than do airports at sea level. One reason is that aircraft engines develop less power in the thin air well above sea level. What is another reason?

Answer 1

-- Airfoils (wings) develop less lift in less-dense air.
So the aircraft needs a longer take-off roll before it can lift off.
Also, its landing speed is greater, even with full flaps.

-- Air resistance (friction) is lower in less-dense air.
So the aircraft needs a longer landing roll before the spoilers
can reduce its speed sufficiently.  (Brakes can't do it all.)