I think it's different because in most sports leagues, the players receive annual salaries, and they have player's associations or unions, so even the lowest level players have a guaranteed salary and the salaries are much higher.
I also don't think they get exclusive contracts where they can't sign with sponsors outside the sport (this might vary in different sports). They might negotiate now in order to help the club employees survive and get paid, but they're paid much more than fighters, and they don't depend on single events to get paid because they have salaries. If a fighter starts training for a fight and it gets cancelled, they're basically fucked, so they're getting into a much bigger risk.
I'm sure that if the PPV points offer was extremely juicy ($5 per PPV or something) Jon would take it. We don't really know what the number is, so it's hard to tell. I think what happens with guys like Jon or DJ is that they feel they are GOATs (which they are), but the UFC only cares about #s and marketability, so it doesn't matter to them as far as pay goes.
It would be nice if the UFC took exceptions with GOAT fighters, champions and superfights and paid them extra for this kind of challenge, unfortunately they only do that with guys with exceptional drawing power. It's too bad because I think they'd still make a bunch of money, but apparently WME is in huge debt and they might not want the word to get out there and have everyone asking for more money, but I'm just speculating.
