My Mac's very secure and I can install whatever I want. But it wouldn't be secure or private to do that on iPhone, because profit.
I think if you don’t get the point of protecting a device against the user, you’ve not had to help un-tech savvy relatives get out of situations where dark or confusing UI practices have got themselves into a mess.
The average user has zero reason to go into some deep settings menu, or even plug in over USB and execute against it to unlock the bootloader.
Unless of course what you install is nefarious?