Why does hentai anime usually suck?

Take a look at some hentai manga or doujinshi.  Regardless of the content, you'll see some amazing art work a lot of the time.
Watch a hentai video.  Terrible, repeditive animations.
WTF?

Yeah, but adult manga is ultra high quality.  The same of detail rarely exists in cel or computer drawn animation, even non-adult anime.