Or buy this and get the exact same thing, but without building and parting it our yourself? It's still an open computer, not a locked down console. The price reflects that reality. It's not subsidized because you actually just properly own it.

The price of this steam machine is a rounding error away from the build it yourself DIY price. It's not marked up, this is just what PC components actually cost these days :/

Interesting claim. Complete parts list please!

Every time I've seen a comment like this, the eventual parts list is about the same price, has large deviations, or re-uses existing hardware (or used hardware). Looking at all the subreddits, the general consensus seems to be the price is fine for the components, and (if you care) it's impossible to build anything with that form factor.

show me a mini PC which has the radiator volume of this steam machine.

actually, show me any PC like that...