Because that is an incredibly excessive alternative. Instead of just duplicating their app to a third-party app store run by someone else, you're assuming that Meta will go to the effort to create an entire ecosystem that they then have to manage. They would have to ensure that each app on the store is on the up-and-up and hire an immense amount of staffing for that sort of thing - moderation, development, backend support, customer support, yadda yadda. Not only do they already struggle to control the narrative around content moderation on their platforms, but now they've gotta work on preventing malware from being installed on folks phones, too?
It's way too extra, when the much more affordable option is to drop a Meta-preferred alternative onto a third-party app store that already exists. Yet they haven't even bothered doing that with something like f-droid, so I'm expected to believe that they're just going to jump to the more cumbersome option of opening their own store? When the average end user will just use the default store that comes installed on their phone?
I don't buy it. And I say this as someone who is very much in the "Fuck Zuck"/"Meta is trash" camp.