You haven't answered my question.
Because it is irrelevant to the discussion.
Why don't airlines get volunteers every single time they have to bump someone?
Probably because they can get away with it. But that does not address the situation where they cannot. Duh.
Sometimes people get away with armed robbery too. That doesn't make it moral or lawful.
Airlines get away with a lot of shit that probably isn't lawful, but that is now routine, and that they expect nobody will call them on - the effort and expense of complaining just isn't worthwhile for any individual passenger, and a class-action would be very difficult to organize.
They expect to be treated as a dictatorial authority, and to be allowed to get away with a lot of really scummy behaviours - and largely they do get away with these behaviours, because they deliberately avoid letting the passengers interact directly with the decision makers.
You have some poor underpaid clerk whose job is to be yelled at by angry passengers who just got bumped, who has no authority or ability to do anything practical to help those people; And then you have the executives who decide to deliberately overbook to get every last drop of profit out of the system, and who write the policies, but who never see a passenger (or at least, not a passenger who knows who they are).
This was the fundamental error made by UA - They had nobody 'on the ground' with sufficient authority to change anything, so when the situation developed outside the presumptions that went into their policy documents, there was no solution that their staff were authorized to implement. There was one perfectly good option - offer more money until someone chose to take it in exchange for their seat - but the staff on the ground were explicitly prohibited from implementing that option, by a policy whose author was not available. That's UA's fault. They engineered a situation where the good (but expensive) solution was not possible, because they were terrified that if they allowed their staff to spend corporate money without a very low limit, that they would be ripped off by their own staff.
They deliberately blocked that 'safety valve', and when the situation reached an impasse, their staff had no options left that were not going to ultimately cause a far worse final result - and so it came to pass.
This was a situation that was inevitably going to lead to disaster for UA; But they were perfectly happy as long as that disaster hadn't happened yet.
The moral of the story is that you must always have someone available who has both the authority, and the ability, to make hard decisions in real situations. Such people are expensive - but they are essential for ANY organization; To operate without them (as most large corporations do) is to set yourself up for a disaster - maybe not today, maybe not tomorrow, but soon. The people who inhabit the boardrooms are very keen to make sure that someone else is sitting in the hot-seat when the music stops, of course; That's MUCH easier (and more lucrative) than fixing the problem.
