It takes a lot less effort to do both of those things, than it is to build a third-party app store and to entice users to join. They may attempt to, but it would be difficult. It’s a lot easier to show ads than it is to get users to use those crappy carrier or OEM bundled bloatware apps. Even Apple has had to allow app hiding or deletion of its first party apps in recent updates, because a lot of users don’t use a lot of them and find them to be clutter.