I wish they would follow Apple's lead and tightly control the hardware. It would make it easier for devs to test and consumers to make a choice. Apple's wildly successful with their iPhone business model. Please try to avoid the fragmentation issues with Android. Windows already owns the home PC market, why go after it?
Yes, I will admit that. Android is king with lots of devices running old versions. Google is trying to wrestle control of OS updates away from the HW manufacturers and Valve has to try to avoid that. Also the choice is SteamBox vs XBOne, PS4, WiiU. Better to have one device to focus all marketing and development efforts on than try the Windows Everywhere tactic.
To add further: there is only one XBOne hw configuration, there is only one PS4 hw configuration, there is only one WiiU hw configuration. Does Valve think it knows better?