Typically it introduces a support and development burden, product fragmentation and a weaker brand. They tried it before (I had one of the Mac clones, it was just ok).

It would eat into their sales but the other side effects are worse, and they would have to look into creating a channel for being able to make money from MacOS.

There is a reason for a number of these boutique Windows workstation manufacturers as there value add is to provide a supported product out of known good tested configurations, because for someone that just wants a machine that works it’s still some effort and Microsoft still gets money for the Windows license.