Section 2.12 of the DVD FAQ says:
"Ironically, computers supported HDTV before settop players, because 2x DVD-ROM drives coupled with appropriate playback and display hardware met the 19 Mbps data rate needed for HDTV. This led to various "720p DVD" projects, which use the existing DVD format to store video in 1280x720 or 1920x1080 resolution at 24 progressive frames per second. It's possible that 720p DVDs can be made compatible with existing players (which would only recognize and play the 480-line line data)."
This is what I mean when I say it's possible to do it, it's just that nobody has yet.