Ah, yeah, good question: why does Apple need to be the one to produce Mac Pro? I could only speculate. Wouldn't it cut significantly into their iMac and MacBook sales if they opened things up like that?
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.