And how do you think that would help them perform? What's your experience in cutting weight on the same day you compete?
I'm asking because I did that once and after that I told myself I'd just compete in the higher class if it came down to cutting on the same day.
Fighters HATE cutting weight, HATE IT; they would not cut and fight on the same day. I can see a lot of dummies in the amateur and smaller orgs trying this but it would hurt their performance i.e. they'd probably lose a lot more than they'd win with that strategy, and so they wouldn't go on to compete in the bigger promotions.
I know fighters hate cutting weight, and that it's the worst part of it. Since you asked, when I wrestled in high school, I never cut more than 3 or 4 pounds, and usually my weight cut involved a light lunch and taking as big a dump as I could prior to weigh-ins. One of my teammates was one of the best wrestlers in the city (Washington D.C.), and he put a shitton of effort into his weight cuts and I kind of marveled at that. That was a long-ass time ago, and I didn't know much at all about the subject when I was a teenager.
Anyway, you're speaking anecdotally here from a single fight. Fighters are still going to go for the biggest weight advantage possible, because it's what allows them to win. Regardless of whether the shorter timeframe means they end up cutting
less to meet that goal, the possibility of cutting far more than they can rehydrate is something that's going to magnify itself, because with less time to rehydrate there will be less time for error, and you'll have a greater chance of fighters fighting dehydrated and incurring additional head trauma, and that's what concerns me.
If fighters were generally in favor of same-day weigh-ins and the UFC and other major organizations were down to regularly test fighters' hydration and weight starting ten days out from fights maybe I'd change my mind, but I don't see that happening within the foreseeable future.