It's because that same escape hatch can be used by bad actors.

And unfortunately as we have seen all too often users are willing to blindly do what they are told if they get something out of it e.g. free game, credits etc.

I mean, it's not luck if you just buy one of those devices...

You don't need a custom ROM to get root on Android, you just have to unlock the bootloader and replace the "su" executable. Moto (aka Lenovo) devices can be unlocked without exploits using their online tool, for example.