Why don't IPTV services like FIOS and U-Verse do the timed DVR recording on the headend side, and stream it down the pipe when someone wants to watch it?

At that point the STB needs to only use the onboard storage as a cache for the the trick-play features (pause/rewind a show you're watching live). And the device can drop into a true low-power sleep when it's not being used, since it doesn't need to keep a disk spinning 24/7 to record shows on its to-do list.

And the provider gets to market it as a DVR with nearly infinite storage space, since it's de-duplicating all the recordings on the headend.