What gives you this idea?

AMD just launched the 9700 and 9700XT earlier this year, with MSRP's of $550 and $600. They've faced a ton of consumer backlash because MSRP cards were virtually nonexistent. Most of the graphics cards actually produced were retailed for hundreds of dollars more, and many were then scalped on top of that.

NVIDIA has been even worse, just cranking up their prices with very little performance improvement. Tons of issues with their power connectors damaging cards. Most of the company's focus has been on AI, and gamers have been left out to dry.

So yeah I think now would be a terrible time for a steam machine. Unless Valve can somehow get their hands on some unreasonably cheap silicon.