Besides all of the trouble it takes to build and run one's own store, pulling one's apps off the official App Store would be hell for discoverability and UX. And the Amazon store really only exists for the sake of Fire tablets.
Ushering the FB app to a Meta store would still require a migration process that would be annoying at best and arduous at worst depending on the user. See what happened to the HBO Max to Max app rebrand this year, except with the additional headache of going to a new app store entirely.
That migration process is the discoverability issue, because now users would be forced to contend with yet another website, marketplace, account, etc.
When someone searches for the fictional new Facebook clone "Visagebook" in the Apple store you can bet they'll want to pay money for "Facebook" to show up somewhere in those results.
