Well because of money. You certainly have to pay to get Ubuntu certificated. And you only do this to have a Linux system with support from the manufacturer.
It’s an enterprise problem with an enterprise solution.
The normal personal systems are not in the same segment.
Why aren't linux hardware shops on Ubuntu's certified hardware list? (ubuntu.com)
Where are Purism, System76, Tuxedo Computers, Starlabs, SlimbookES, and others? Instead there’s Dell, HP, ASUS, and Fujitsu…