It is entirely possible to lock down computer parts to only run Windows and signed drivers. However, the sheer amount of available computer parts, open sourced hardware, widely understood technologies, and not enough monopoly makes this unfeasible for anyone to really try to implement (yet).

If Intel started doing Windows only, they would lose so much revenue from big corporations and data centers it would ruin them, and everyone would just buy AMD instead. Consumer market in computer sector is secondary.

For phones, you really do not have enough alternatives. You choose between evil and more evil. Think of it as Linux in it’s starting days - missing features that makes it unusable for the common folk. Linux phones haven’t matured yet, that’s why you have to choose between feature rich vs heavily degraded user experience, as opposed to minor inconvenience of not being able to run some apps.