Why are there no fully networked DVRs?

In the past I have owned HD DVRs from TIVO and Comcast. Both of them are decent DVRs if all you care is to record, stop and replay. But they do not network. First of all Comcast has no networking capability. TIVO has very limited primitive networking capability to one single PC of your choice on your home network. It also needs a high speed wireless network, and it does not connect to to the Gigabit ethernet of my home LAN. So I started doing web searches for networked DVRs (I want an HD DVR though.) I came up empty. Apparently no company offers products along these lines. Do you know why? Is it purely marketing? Or is it related to any regulations from FCC? Or something else? Will we see fully networked DVRs in the near future, who can talk to all computers and all NAS (network attached storage) on a small LAN?