Why do scientists publish so much? There is the whole "publish or perish" thing but that doesn't explain the large increase in quantity of publications. With a focus on important publications and measures like H-indices, a simple publication in a conference often won't help someone's career. Yet we continue to publish as much as we can. Why?
When you play a slot machine and you win, you get lights, music, sounds of coins coming out of the machine. If you lose, nothing. Lots of positive feedback if you win with no negative feedback if you don't.
If you submit a paper and it gets accepted into a conference, you feel excited. Excited to see your name on the list of accepted papers. Excited to update your CV and to give that talk or poster that only those few who's paper was accepted get to give.
If your paper is rejected, nothing. You don't list rejected papers on your CV. Nobody will even know you submitted the paper. And you can just take that paper and submit it to another conference. Lots of positive feedback if your paper is accepted with no negative feedback if it isn't.
Some people might argue the analogy doesn't work since slot machines are arbitrary and random. Those people have never seen a program committee in action.