Mostly because very few people (if any anymore) see a movie based on the poster so why spend any real money on it?

Unfortunately this is very true movie posters and other printed adds where a lot better when that was still one of the main ways to attract viewers/new customers.  Movie posters started to become largely irrelevant as a mayor form of advertising for new movies in the late 90's.

The majority of people know what they want to see at the movies before they ever leave the house based on trailers. etc. But it does seem they could do a bit better than those. I could have made better ones in 10 minutes. I probably would have been much cheaper too lol

The Social Network is such a great film, everyone should check it out.

If you have a limited budget, at least try to be creative for what you got.

I thought people might like to see this:
http://imgur.com/gallery/jrmWj

Surprised they didn't mention titanic in that last one, it was obvious

Some really good movie posters by the late great John Alvin in that link.  Its also sad that Richard Amsel died so early the last movie poster he did for Mad Max Beyond the Thunderdome is also one of his best posters and one of the best posters from the 80's.