Motherboard and CPU, memory, NVMe drives, PSUs, SlimSAS cables and breakouts, a custom machined case, assembly, support.
You're free to try building one yourself for cheaper. If you consider your time for researching/assembling/testing it to be worthless, and are happy with a contraption in a miner frame, then you can probably do it.
PC builds seem to short circuit everyone's pricing logic and drive any labor cost down to $0, just because they're willing to do it for free. Anything above that $0 is considered overpriced.
There are services that will build a PC for $200. It's entirely valid to ask where the money goes, and the answer is not the labor to put the pieces together. There's no reason to assume OP is being dismissive of that specific cost.
You're free to try building one yourself for cheaper. If you consider your time for researching/assembling/testing it to be worthless, and are happy with a contraption in a miner frame, then you can probably do it.