Most guys do fight often, until they become Top 5 in the UFC. Getting paid for things like fan meet n greets and turning up to conferences gets in the way of training IMO, its all $$$.
Then when they need to train for a big fight, they go HAM trying to regain the physique and fitness they had when fighting 4, 5 or more times a year and injure themselves.
Speculation I know, but these guys are getting much more money than we're led to believe. I'd love a 4 fight per year minimum in the UFC for every fighter, thats one fight every three months barring injury. Of course with the amount of injuries, such a rule could not be kept, but it would mean everyone is training to some extent at all times and there SHOULD always be a replacement fighter ready to go.