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.)