They wouldn't have needed this expensive hardware if they went with the same or similar AMD hardware as in PS4/X1 and take advantage of their Mantle API, which basically makes the system as lean and efficient as a console. Not to mention it would be trivial to port the same highly-optimized console games to the Steam Machines. Carmack thinks Mantle could help Steam Machines with AAA titles a lot, too:

http://www.tomshardware.com/news/amd-mantle-api-xbox-one-pla...