The unlocked bootloader of PC-era was just a quirk of history
The PC had an unlocked bootloader for the same reason the Apple II had an unlocked bootloader: no one knew what the hell they were doing.
The PC era launched with a multitude of operating systems. No one knew how a PC could or would be used.
Contrast to our mobile era. While I’m not convinced we know how mobiles would or could be used, it is clear that contemporary manufacturers are students of history.
PC-era IBM and Apple both made the same mistake on their hardware platforms. That mistake traded potential profit for revolution.