Why isn't it like this already?
Are they avoiding malicious apps running people's phones like say Limewire and Kazaa files back in the day?
All Apple needs to do is be ok with other App Stores.
https://www.congress.gov/bill/117th-congress/senate-bill/2710/text
Discussion
That's what Apple would tell you. My guess is that it has more to do with
$$$
Why share the pie when you can eat it all?
Then again Android is FOSS and it's like this now.