But today the vast majority of tickets are non-refundable. Most airlines keep at least $200 of your domestic fare if you cancel or don’t show up, and $300 or more on international tickets; some fares on some airlines(especially these new “basic economy” fares) keep all of your ticket price and then they resell your seat to someone else, usually at a much higher last-minute fare. So now even if they don’t resell your seat by overbooking, they still collect revenue. It’s greedy to expect to get paid twice for the same seat.
Furthermore, some passengers who get involuntarily bumped suffer real hardship that goes way beyond missed vacation days or the cruise they weren’t able to catch. What about the missed funerals and weddings or job interviews? Airlines bump people not based on how urgently they need to get to their destination but on frequent-flier status or whether they’re in first class or coach. The process is often capricious.

USA Today