I can only think there are DRM issues involved in streaming to a PC - which is more open than an XBox 360 or hardware extender - or it is a legacy of MS trying to protect the extender environments.
As the 360 is a non-starter for many as an extender (too noisy) - there has to be a different solution.
At IBC in Amsterdam last September, MS were promoting the networked home with Media Center stuff, and they now make sharing of Recorded TV across networked PCs trivial in Windows 7. Surely streaming of video, audio, subtitles and interactive data can't be that difficult? (Sage, Myth, GBPVR etc. all do it...)