They're not, because they don't lock down the hardware to only Steam.
If they subsidized the cost, people could just buy them as general purpose computers and not buy steam games on them.
Valve would only be in a position to subsidize the hardware if they locked the hardware down to just the Steam store.
> The National Center for Supercomputing Applications had already built a cluster based on the PlayStation 2.
Not to mention that the NSCA was just throwing spaghetti at the wall to see if it would prove useful when it came to the PS2,[0] and their setup never worked reliably.[1] The PS3 had several supercomputers made independently.[1][2][3]
[0] https://www.ncsa.illinois.edu/2003/05/27/playing-the-superco...
[1] https://www.warhistoryonline.com/war-articles/ps3-supercompu...