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?

Please dont over generalize that all hentai sucks in terms of animation. These are some of the hentai movies i've seen that have excellent animation:

1. Tempation
2. Midnight Sleazy train
3. Angelium
4. Immoral Sisters 1 & 2