The Steam Machine is "just" a normal Linux PC, with good integration.
The point is the target demographics. And, just like the Steam Deck, this is not something for power users. Most people on here know how to build a PC and install Linux with KDE on it, which is the same experience you would get with a Steam Machine.
Valve is selling this to an audience of gamers, specifically those who want an easy gaming experience on low end hardware. This is the same demographic which Sony and Microsoft are selling to.