You’re still not understanding: people buy stuff for convenience. Buying a console which you can play all the latest games on with a controller is convenient.
The target audience for Steam Machine (or whatever they’ll call it) is not you or some linux dude who like setting things up themselves, it’s people who just want things to work ™.
What you’re saying is “Why would I buy bread when I can make it at home? Nobody’s going to buy bread!”.
I guess we’ll see 🤷♂