Spot on comment! One thing that completely gives away what you said is that these girls do upload 1080p or even 4K content on their YouTube channels. I think a perfect example of this is Lauren Alexis. Her YouTube content is in 4K, horizontally shot, nice lighting, she's fully in frame, clear image, and shows off her body and ass perfectly.. yet her OF content is all low res, has weird filters over it, has poor/low lighting, is vertically shot and you don't even get that many good looks of her body and ass and she's not even fully in frame a lot of the time.
So there is 100% something going on here and I've always said that OnlyFans is just one big scam and a get rich quick scheme. It came out of nowhere in 2016 made by some f*ckboy, suddenly every girl with an online platform was doing it (even the once who were against sexual content or the ''objectifying of whamin''), there is no regulation to speak off, the same goes for price/quality ratio, no preview before you buy (that's why leak sites are awesome as they show the products); and let's be honest here, most OF content sucks and is low effort content.