The UFC has more power than just stripping them. They are under contract and I can all but guarantee that it is in their contract that they aren't allowed to fight in another promotion, including boxing. ****, I don't think they are even allowed to do WWE without the UFC's permission.
You're right that other fighters aren't making Conor money in the UFC either, but I think we can all agree it is not good for the UFC to tie up one of their champions (especially in a division like LW) for the better part of a year just to watch him go get his ass kicked in another sport. The UFC's cut of the revenue would have to make it worth it and no other UFC fighter is coming close to generating Conor revenue in MMA or boxing. So I don't think the UFC would let them do it.
If they did and they stripped them, the fighter
wouldn't be champion when they come back to the UFC, so even if they did get an immediate title shot, they wouldn't be making as much money as their previous MMA fight. They probably wouldn't do it to a 29-0 Khabib, but they could make it even more difficult on them if they wanted to and say they even have to take another fight before they get the title shot since they were out for so long.
So there's a lot of hurdles to jump to make something like that work out. Everybody knows and kind of accepts Conor is an exception given how much money he generates. But if you let other guys do it, I think it's opening a big can of worms. So as
@m9 said, it's probably not ever going to happen for 99% of the the UFC fighters, which makes it even more annoying that they keep talking about it.