Valve is likely already selling it at a loss. There's a limit to how much loss you can swallow, which is why the Nintendo Switch 2 is $500 and not $300. There is also a limited supply of components; the more you subsidize, the further the price drifts from the supply/demand cost of the components. At $1,300, Valve can sleep easy knowing that the console can still be made and sold in low volume to the people that want one. Subsidizing that price to $999 or $899 would be unprecedented, and force Valve into a hardware shortage until they can secure low-margin components.
The Steam Machine as a hardware product will probably be a Playstation 3 situation. Comically unaffordable at launch, but iteratively updated with lower-cost SKUs that convert curious customers.