From a full-system integration might not be cost ineffective:

    1. Single SKU for Server & Client (volume reduces cost)
    2. Games target & optimize for this hardware already (nothing new) 
    3. Microsoft & Sony are essentially *the* targets (no platform integration cost)
    4. Proven user adoption (approximate ROI known for these projects) 
    5. Brand recognition (xbox and playstation are household names)
    6. Price (Microsoft is doing subscription for service + catalog)
    7. Bought publishers (Microsoft & Sony "own" many dev shops)
    8. Primary library vendor (DX11)
    9. Network Effect (I bought an xbox because all of my friends owned one)

The only new components Microsoft needs:

    1. Mount Game (iSCSI)
    2. Output (HDMI to RTMP transcode) 
    3. Input

I'm not saying either solution is better (I prefer Google's approach) but they're only doing it if they've run the numbers.