Because being champ at Glory for example, like Verhoeven gets you paid more then being champ at the equal UFC weight class like Stipe Miocic. If not, they'd be trying to get to the UFC. Plus there is the added bonus of knowing that even if it is more streamlined skillset, that because of the specialization in skills, and the fact that you can be number one in a sport were there is more guys actively participating in right now (and all time), means you get to feel better about yourself as an athlete which could matter in a small way, but I will accept that that second point is a bit more of a stretch.
Sort of related in a roundabout way, but relevant, is in the Mayweather vs McGregor shitshow, Verhoeven was there to watch as a spectator. It should tell you enough right there, that he was either important enough to somebody, to invite to the event, or he had enough spare change to pay for the ticket himself just because he wanted to go.
Any MMA personality there, was there because they were working the event in some capacity, and that is considering they didn't sellout the live venue, and the UFC was a minor promotional partner in the event, and they still couldn't get a couple of guys in there.
