Uhh... well, the airlines schedule the flights at time when people actually want to fly. It would really cut down on congestion if airports spread out the flights onto a 24 hour schedule, but not many people want to fly from Dallas to LAX at 3:12AM.

Spreading out the schedules would be less efficient for the airlines, and it would just mean longer layovers for pax. That's not an improvement.

And good point yogi - the on time record is based on the time the flight arrives, not departs. Who cares if the flight departs late so long as you get there when you planned to?