Why would we want a subsidized Steam Machine?
Subsidization pretty much requires that the hardware be locked down and walled-gardened so the subsidy they put into it eventually pays off. The Steam Machine, currently, lets you install whatever OS you want and use it like a regular PC without locking you into the Steam ecosystem at all.
"Okay, so they should make a locked-down subsidized one and an unlocked full price one" someone might ask. That's extra development and support work they have to put into it, and it would only be a matter of time before someone jailbreaks the subsidized one to work like the unlocked one.
Plus, you can make any capable PC a "Steam Machine" since they also give away Steam OS. My DIY Steam Machine runs Bazzite, but I was close to putting SteamOS on it. The only thing that made me choose Bazzite was that pretty much all of the patches and special module to support this former AMD crypto-mining card were already baked into Bazzite.