that's the thing with some of these cans (probably not all of them, but at least a few of them). What they do is actually relatively low risk compared to serious boxing. I'm sure a lot of them don't train, at least not a lot, so there's no risk of gym damage (which is in my eyes the source of a lot of boxing damage). They sign up for a fight every few weeks to make a quick paycheck, probably don't care, go down without too much of a fight. Usually they face equally talentless guys, so the damage is generally fairly minimal.
Reggie Strickland is famous for this. 66-276-17. He was fighter literally there just to lose. He was once asked how he trains for his fights and merely responded "I don't." That's the attitude of the guys I'm thinking of. Fight other cans for relatively low paychecks, don't take too much damage, supplement another job you have. Maybe an up and comer for a bigger paycheck. Remain a local favorite, fight at the bar every once in a while for a free meal and a few hundred. Is it physically damaging? yes, for sure, but for guys with education as poor as a lot of these guys, ALL of their alternatives are physically damaging. Construction worker, miner, factory worker, whatever. It's all dangerous. These aren't guys with PHDs who chose low level boxing over a job at wall street. These are guys who fight the occasional fight so that they don't have to work two jobs, and with minimal training required, the bodily damage is probably nowhere near as much as top level boxers.