Earlier quoted context omitted.
I also think selecting parts is limited by the fact that there are so few manufacturers. Processors these days mostly come down to AMD or Intel and nothing there is transparent or audit-able. "trusted computing" and backdoors make even your hardware suspect.
There are capable desktop computers with open hardware, down to the silicon: you just have to pay for it. For example, for $4k, you can get this with specs roughly equivalent to a normal developer machine: https://www.raptorcs.com/content/BK1SD1/intro.html
I bought two of these last year and they're great, stuff your own memory in there, add some storage and off you go.
Edit: clarified that this would be a mainboard + CPU.