why can't we watch HD movies from DVDs?

data rate.

a DVD movie is 6 mbps. a blu-ray movie is 24 mbps. blue-ray also uses higher compression codec (AVCHD) which requires even higher speed decoding. DVD movies use MPEG2 which is the same as digital television so it uses a much less expensive IC chip set.

you can record HD on a DVD but because of the data rate difference, only 20 minutes will fit on a DVD disc. newer models of Blu-ray players can play DVDs with AVCHD encoding. However, don't accidentally put such a DVD with HD video in a regular movie player, it will damage the disc, the player or both. For computer archival purpose, you can put HD encoded files on a DVD-rom, but they won't playback in real time, and again, the 20 minute time limit applies.

Because of the settings and the way it's designed.

It has certain video codes writtten on it, which can only be decoded by a video player as normal quality and not HD quality.

Same reason why you can't watch a DVD through a CD, or a CD through a cassette.

They are... if you really want to get down to it... different formats. I'll bet the purists would support this idea.

Aside from memory? Memory is it. That's the one reason.