That's because they haven't come out yet. One of the main complaints with PC gaming in regards to console gaming is ease-of-use. This is what the Steambox is trying to solve. I imagine that you'll be able to see on Steam whether or not your Steambox will be able to run a game and how well it will run. Not to mention casuals won't have to build their own PC as it's already pre-built.
Nonsense , steam machines are still going to come with a wide range of hardware and all the inherent problems this brings.
The SteamOS isn't going to magically solve the hardware variation problem.
These boxes are still going to have all the jank that comes with PC gaming.