As for the why, have you ever tried installing vanilla Windows on a modern notebook? I know Linux has a reputation for bad driver support, but in my experience with Dell, Lenovo, and HP, a vanilla Windows installation will have no graphics driver, no network driver (neither wired nor wireless), sometimes the sound drivers are missing. It is a shitshow. Meanwhile, Linux on the same machine generally works out of the box.

But the answer to the question "Why do OEM versions of Windows even exist in the first place" is that without OEM customization, a Windows computer is a non-functional computer.