I also think that a lot of the remakes are because movie studios know that people are somewhat nostalgic and will go see a remake in order to compare to the original. They also benefit from a "new" audience.
Just look at the "superhero" movies. They they really have to reboot batman, superman and spiderman?
I'm pretty sure that studios also want the low risk approach given some of these movies are so costly to make. They figure if it work once it will work again. Same applies to video games most of us play. Look at the most popular ones. Cod, BF, GTA, Skyrim ...