Don't you think it's an alternative professional football league for those that can't make it in the NFL? It's certainly not a developmental league -- that's the state of FBS NCAA football which has become a billion dollar industry & feeder system for the NFL.
I'll focus on QBs: Almost all seem to be low drafted or undrafted prospects that couldn't make it in NFL but want to continue to play professional football. Take Max Duggan for example. Drafted in the 7th rd by LAC & released after just one year where he simply doesn't have a "NFL caliber arm." Instead of going into coaching or getting a job in his college degree field, he gets to play professional football because of the UFL. If he thinks he's in the UFL to develope a NFL caliber arm, that's not going to happen.
NFL caliber arms are usually seen already at the HS level, hence your 4 & 5 star recruits (it's similar to these HS pitchers throwing in the 90s that get attention from the MLB scouts & top college baseball programs). These college QBs either have a NFL caliber arm or they don't (accuracy, ball placement, trajectory, etc, are all elements that can be developed but not arm strength). And there can be starting QBs at FBS schools that don't have NFL caliber arms but a good enough arm to play at that level. I see this all the time at Wyoming, Colo St, Air Force, etc - some of the lower-tier FBS schools in my neck of the woods (other than Josh Allen, when did a Wyoming QB ever make it in NFL?).
So, if the league continues indefinitely, it provides an outlet where these NFL QB rejects can still play football & get a little dough for it (probably better than working a 9-5 or trying to start a successful YT channel. Lol).