Being available for bumping means you have the time at the other end of the flight FREE! Most people have to work or are working. Wolunteering is another luxury of the upper percentile in any country.

I want to get bumped from Business Class on NW in NRT on the way from BKK to LAX, so I could sleep in a real bed, have a good meal, and shower with the good rest. Starting over fresh for the last leg out of NRT. Those 6AM flights out of BKK mess me up. Got on the plane SMOKING(a mistake in seating) section, got off, but was put back on in a non Smoke section of Business class.
I would sleep over at NRT on any trip if it were possible.