Why, instead of safely entering a BIOS setup

effiency and lawsuits, phones has embedded hardware, its a bit op to have that initial hardware calls for a embedded hardware system.

BIOS is initally an IBM tech

_does the cell phone brick when installing the Custom ROM wrongly? _

Android is based on linux, that includes the partitioned bootloader (mostly grub on linux and fastboot on android, they’re not technically the same but the idea is somewhat related) if that partition is messed up then its most likely not to boot

Wouldn’t this protection be better for users? I mean, this could be done through ADB.

Android is owned by a corporation, I dont think that will be their primary objective

Also, do you think it’s possible that this way of doing things will come to the computer, with ARM hoping to gain a good share of the market and all?

ARM is mostly a cpu design corporation that offers license fee to other companies to manufacture thier cpu designs, they’re everywhere. It depends on thier licensees what to add to make profit.