Because at some point, Valve is about making money, and people don't buy consoles and attach them to their tv with mouse and keyboard en masse. If they did, this wouldn't exist.
If you want/need to sell something in the millions, it kind of needs to be least common denominator. Gamers understand controllers as the basic interface for tv gaming, so they will buy them.
Gamers don't buy keyboards to put on their TV's so Valve is being smart by not forcing a square peg in a round hole. They want the Steam Machines to sell. A mouse and keyboard won't sell.
If you want/need to sell something in the millions, it kind of needs to be least common denominator. Gamers understand controllers as the basic interface for tv gaming, so they will buy them.
Gamers don't buy keyboards to put on their TV's so Valve is being smart by not forcing a square peg in a round hole. They want the Steam Machines to sell. A mouse and keyboard won't sell.