I think it's just not worth it for them; look at Android, where sideloading was always available as an option.
Facebook does offer separate APKs on their website (so do, in fact, most major services - Netflix and Spotify also offer APKs from their website), but practically the only reason people end up using them is if they're on a device that doesn't support the Play Store (for whatever reason).
The only serious Play Store competitors on Android are either vendor specific (like Amazon's store) or wouldn't host Facebook apps to begin with and are unambiguously a positive force for users due to their standards (F-Droid, whose policies are designed to protect users from the typical mobile app rugpull of filling it with ads down the line). Anything outside of this tends to be independent hobby projects or corporate business apps.
The inertia of being the default is still really strong (for a slight alternative, much of Google's strength comes from the fact they paid millions to browsers to be the default search engine for them, a practice that's been found to have violated antitrust laws - it's telling that Google really wanted to keep doing this), which is still enough to keep Facebook attached to the Play Store and is probably why they won't try to leave the App Store either.
Facebook does offer separate APKs on their website (so do, in fact, most major services - Netflix and Spotify also offer APKs from their website), but practically the only reason people end up using them is if they're on a device that doesn't support the Play Store (for whatever reason).
The only serious Play Store competitors on Android are either vendor specific (like Amazon's store) or wouldn't host Facebook apps to begin with and are unambiguously a positive force for users due to their standards (F-Droid, whose policies are designed to protect users from the typical mobile app rugpull of filling it with ads down the line). Anything outside of this tends to be independent hobby projects or corporate business apps.
The inertia of being the default is still really strong (for a slight alternative, much of Google's strength comes from the fact they paid millions to browsers to be the default search engine for them, a practice that's been found to have violated antitrust laws - it's telling that Google really wanted to keep doing this), which is still enough to keep Facebook attached to the Play Store and is probably why they won't try to leave the App Store either.