From the Cranky Flyer blog:

"On the whole, the airlines make more money overselling flights than they lose paying out compensation. And this actually does help keep fares lower. If airlines couldn’t oversell flights, they would generate less revenue overall and have to find a way to recoup those costs. You can connect the dots on what that means. Of course, if enough people were impacted that the pain was too great, then either the airlines or the feds would do away with overbooking. But considering how many people benefit from being on an oversold flight and volunteering to get paid (over 550,000 in 2015), it’s not something that a lot of people WANT to go away."