Apple’s App Store has very strict privacy rules. Last year Apple implemented the App Tracking Transparency requirements, which Facebook says will cost them $10B in lost revenue this year [0]. If sideloading becomes a thing, I can definitely see Facebook requiring it in order to get around these privacy rules.
That's a dangerous gamble. The data tracking and privacy concerns aside, users are going to be mad at the additional friction that involves. They might get mad at Apple for making the enablement of sideloading an uneasy process (lots of disclaimers about how insecure it is, etc.), but they'll also get mad at Meta for pulling their apps off of the store for no discernible reason. Meta would have to offer more than the existing service they're getting from their current apps, to convince users to do this without losing goodwill.
[0]: https://hothardware.com/news/facebook-claims-10b-revenue-hit...