What is clear is I’m getting performance per dollar spent in a far better ratio than if I buy one of those.
What you may not be understanding is that those are among the highest-performance PCs out there, and you can't build them for cheaper than you can buy them from an OEM. You may be able to get more performance per dollar for much lower-performing systems, but that's it, they simply won't be as fast. Dual-processor always costs a lot more.