It's my damn phone!
Correction: it should be your damn phone. As it stands, it's their damn phone; they're merely being gracious enough to let you use it in the limits they prescribe. And you're paying for the privilege!
I'm not going to install some random piece of closed source software that couldn't make it through Apple's review process. No I'm not treating my phone differently than my computer. I have no random software on my computer from untrusted sources. I even bought my last Dell from the Microsoft store so I wouldn't have random crapware on it.
Doesn't sound useful to me.
Can you name one successful profitable Android app that isn't sold in the Google Play Store even though Android devices have been able to sideload for years?
A lot. Your mistake here is in thinking that all software is direct to consumer. Being able to install software without going through a store is very important for a lot of small to medium business that have custom or customized software. You can do it with android, you could do it with windows mobile in the 90's, it's mostly just apple that want to exert so much control over you.
https://developer.apple.com/library/content/documentation/ID...
... Yet Microsoft's retail version of Windows 10 still comes with PUPs, like the nigh-impossible-to-uninstall Candy Crush Saga.
You would assume then that Microsoft must have well-vetted CCS. However, it's been a vector of attacks, including a time when it dumped a trojan out during it's reinstall process.