PCs have huge market penetration yet lag behind consoles in dev interest (a lot of PC gaming revenue is microtransactions in web-based games). Steam Machines are starting from scratch (or around 2% or whatever Linux desktop market share is), so I doubt many developers will want to spend time and money porting their games to Linux.
If you look at Steam Machines suggested minimum specs (i5-4xxx, GTX 660, 16GB RAM, 1TB storage), the price is unlikely to be below $400 or even $500 (someone will be trying to make money on them), and I don't think that many people will pay that kind of money to be able to play games that are now available for Linux and have uncertain developer support for future games.
Yes, Steam Machines will also be able to stream games from existing gaming PCs, but $400-500 is a lot of money for what can be done with a couple of longer-than-usual cables.