They do have their own App Store, it’s just only got like… 14 apps in it. Mostly just the stuff they’ve made them selves that they’re super confident in the security of, as well as a couple of other app stores like Accrescent

To start allowing submissions of any third party app to it would ether require them to do a ton of vetting to ensure it meets their standards, or for them to drop the standards for security and privacy for it. If you want more than that handful of defaults, Accrescent and the google play mirror are there ready to be installed

They already have one. Maybe they could partner with F-droid...