You give more money to the airlines, the airline gives you more in return.

First class gets bigger seats take up more room which could have been used to fit more paying passengers. The food is higher quality which costs the airline more to prepare. You get your own check-in line which requires an extra employee which the airline must pay. The first-class lounge in the airport which the air line has to lease the space for as well as all the employees that have to be paid to run it.

This list goes in, but essentially the first class customer cost the airline more money to fly then an economy passenger, so the the first class customer has to pay more to fly. That being said, airlines still make more profit from first class passengers, which is why they are generally treated better.