I think a large part is because Hollywood is running out of ideas and it is easier to retread an old idea than come up with something new. Rarely a remake is better than the original. "I am Legend" was much better than "The Omega Man." Some remakes are so audiences can better relate to classic stories like "The Magnificent Seven" being a Western remake of "The Seven Samurai"
Some remakes are good but still pale compared to the original. The remake of "Hairspray" was a good movie in its own right but it lacks the edge of John Water's original. I've never seen the new "Pink Panther" because I have such fondness for Peter Sellers character. I've always liked Steve Martin and I'm sure the remake is good but Peter Sellers was such a comedy genius that I don't think anyone can fill his shoes.
It has been proven that people that are fond of a movie that came out when they where young would like to see how the remake looks and compares. While at the same time there are many people that never got to see the original at the movies and want to see what the fuzz is all about.
Financial reasons also takes part since the original movie was already made and they can get ideas of what worked and what didn't.
Another reason is that when older movies came out there was no CGI and some special effects that looked bad then, can be made with more realism now.
Is a fine line between what movie gets remade. In my opinion there are some movies that are so classic that making a remake would be an incredible risk because it will be compared to a classic.
they can't think outside of the box or they're wishing that they would the same or more amount of money made from the movie or they're all out of ideas that fit in with our economy because the really good ideas are really expensive? i'm just taking a wild guess
Maybe these film makers want to reflect the "history" or the old story from the new point of few. They want us to see that one "sentence, same sentence" can be visualized differently. I think it is challenging...