It has always been like that since CP/M and commercial UNIX days.

Vertical integrations like everyone sell a product, a brand, a whole ecosystem experience.

If all OEMs sold the same CP/M, UNIX, MSX, MS-DOS, Windows software stack, on the what is basically the same hardware with a different name glued on the case, they wouldn't get any brand recognition, aka product differentiation.

Thus OEMs specific customisations get added, back in the day bundled software packages are part of the deal, nowadays preinstalled on the OS image, and so on.