Like how you can't get Facebook on the Play Store because it's only available on F-droid on Android, for example?
That's certainly... interesting.
Facebook is quite happy on the Play Store because it has weak privacy and disclosure policies, because Google has no interest in enforcing such things.
So there may be a good reason why Apple don’t want this to happen, given that essentially everything is on your phone these days.
There’s still Android
Yes, this is why the phones are locked by default. This is not at all why the phones aren't unlockable, though. They can be unlockable and still be secure. Apple just doesn't want to give you the choice.
Really secure data goes through a seperate device of which I have total control, but which I treat carefully (and less conveniently). It's a very comfortable trade-off.