IBM didn't think to lock it down, the BIOS was the main blocker and was relatively quickly reverse-engineered (properly, not by copying over the BIOS source IBM had included in the reference manual). They tried to fix some with the MCA bus of the PS/2 but that flopped.
> almost every phone has closed drivers
Lots of hardware manufacturers refuse to provide anything else and balk at the idea of open drivers. And reverse engineering drivers is either not worth the hassle for the manufacturer or a risk of being sued.
> Why are there not yet a plethora of phones on the market that allow anyone to install their OS of choice?
Incentive. Specifically its complete lack of existence.