Hollywood eats itself. Always has - there were remakes back in the 20s, even.
These days, I think there are two things going on-
1, Title cache: People know the name, but never saw the original movie. Hence the tentpole horror movie remakes like Halloween, Texas Chainsaw, etc (in addition, horror movies are often cheaper to make and draw a teen audience).
2, Fan W@ank: The kids who saw the original during their formative years (either at the cinema or on TV) are in the position to make a low-risk remake (hey, it'll make money on DVD, right?) rather than come up with an original idea that might not work.
Prequels are another fruit of this poison tree- I just watched the Thing 'premake' - how pointless is that movie to fans of the Carpenter movie? We needed to see what made Hannibal Lecter a cannibal? Or why Michael Myers was a kill-crazy nutjob?
I'm waiting for the prequel to Jaws, where Quint first encounters the same great white after the sinking of the Indianapolis, and the entire backstory of how the great white was mistreated by it parents.
Ya know what? Scew remakes - I wanna see some crossover movies - the killer in Saw is actually Macauley Kulkins character from Home Alone all grown up, Pumpkinhead is what evil Gremlins grow up to be...
How about spin off movies completley unrelated to the original? 'King Dog', about the trials and tribulations of a 1930s hotdog vendor in New York who tries to make a new snack available to the public - made from giant monkey meat.
C.