Why do you pay more in taxes on the second dollar you earn compared to the first? It's an acknowledgment that flying is a useful activity for many people, but still an incentive to minimize unnecessary flights.

I think part of the motivation is to not punish infrequent fliers.

I think it's a little like how utilities price water. Water is very inexpensive at first. But if you are filling a pool and watering your lawn a lot, the gallons get more and more expensive.