Fedor and other guys like Sergei Kharitonov have mentioned how terrible UFC contracts are and why they'd never sign them.
It's been awhile so I'm paraphrasing but they mentioned how the UFC owns your likeness, image, owns your fights, tells you where to be, what to wear, who to talk to and doesn't let you do anything related to fighting. On top of that, Fedor specifically mentioned how Dana is disrespectful, doesn't like his attitude and wouldn't want to work for him.
If you are someone like Fedor and UFC offers you 1 million dollars but owns everything you do or, Bellator offers you 800k and your only obligation is the week of the fight AND respects you, the choice is obvious.
A lot of us have a problem thinking with a fan's mindset rather than the fighter. Fedor doesn't care about fighting in the UFC, being the GOAT or winning the UFC title. That's a fan's fantasy and something they want to see. Guys like Fedor and Sergei just want to be paid, fight and be respected and they don't need the UFC for that.