Especially when the reason for the move is obvious. If Facebook leaves the app store, and Facebook is completely free and makes money from advertising, everybody knows why they're doing it and it's to screw you out of your privacy. But you can't say no because Facebook is the only way you have to talk to your grandparents, hence the need for antitrust.
If Epic Games moves Fortnite to another store because the other store charges lower fees but still has a good reputation for not allowing bad apps, likewise the reason they moved is obvious. It's to have lower fees, because Fortnite makes money by selling things in the game. Many users will be fine with that, as well they should be, because that's the point of allowing other stores.
But a game doing it for the same reason as Facebook, well, they're going to lose most of their users. Because nobody needs a game that much, so the company doing it for an adversarial reason will cause large numbers of customers to tell them where to go.
The primary reason for this is that Facebook itself has a dominant market position (network effect) and is deserving of antitrust scrutiny. "We need this evil monopoly to protect us from that evil monopoly" has a better solution in smashing them both.
Notice that this argument doesn't work anywhere else. If your tetris app which is in competition with many others decides it wants to go out of Apple's store so it can spy more, nobody is following you. They'll just install one of the hundred others.
To gain anyone's trust for any app that isn't a monopolist, an alternate store would have to have a similar reputation for rejecting bad apps. But then the scammers wouldn't be able to make it into any of the stores people trust. The advantage is that some of the non-scammers that Apple rejects for illegitimate reasons, would. (Or competitive pressure would cause Apple to get better at not rejecting them in order to prevent it.)