George Lucas started this trend when he went back and updated scenes from Star Wars.
A lot of people like to say its becasue Hollywood is "out of ideas", but that's ridiculous -- the potential for new stories continues to be limitless. I think the real reason for a lot of these remakes is evident in the specific movies that are being remade -- movies with lots of special effects from the 80's and 90's (e.g., there are remakes planned for Red Dawn, Mad Max, RoboCop, Judge Dredd, Escape from New York,
and one of my all-time favorite movies, The Crow.) The fact is, today's technologies offer the potential for the movie to come closer to the original "vision" of it, a vision that was compromised by available technologies of its time. It can work or not, and fans of the original will often have a visceral reaction against the new version, but objectively, its a good business move by the studio: a heavy-on-the-special-effects brand with existing name recognition that could be improved by advances in effects.
But that said ...
they better not f*ck up The Crow!