Their SOC makes sense in the Steam Deck because it's running at a lower resolution. If you try to run at 1080p, you'll get significantly worse performance, and that's kind of the bare minimum resolution for a console.
Valve would need a significantly stronger SOC to run on a home console, and it would likely need to be stronger than what's in existing consoles because they don't get the benefit of devs targeting that SOC.
I'm guessing they'd need to sell for ~$800 for it to make any sense. That's quite a bit more than existing consoles, but it's a competitive price for a gaming PC. And I'm guessing the market for a Steam console is a lot smaller than a handheld.