Like I said yesterday, when I went to the movies I had plenty of time to waste before the movie started, and I couldn't help but noticing the movie posters that were advertising upcoming movies... not thinking about how much I wanted to see them, but what other covers they looked like (and yes, every movie poster really looked identical to numerous others).
Movie posters, the same as book covers, attracts the attention of the viewer/reader, and all these similar looking covers tells the viewer the genre and something about the movie that they might want to see. It was easy to identify what sort of movie they were based around the cover, because they able to relate and capture the attention of the audience from another movie.
It's not just movie posters that use identical features, but book covers do this also. The cover tells the genre, and a lot of books use similar features to identify what the book is about. But, this is a good thing because you know what you're getting yourself in to (and a lot of the cover artists knows how to make them still look original and elegant while still using similar features of others).
Have you ever noticed how much covers/posters look the same? Do you pick up a new book or go to a movie because the cover reminds you of another one you've seen?