Actually... The real truth behind this is twofold. One, it's often cheaper to purchase the rights to remake a film than to buy a quality script from an established writer, espeically when the studio shooting the remake already owns the rights to the film... then the cost becomes negligible to do a re-write. Second, and probably more importantly, is "Brand Recognition"... everybody knows the name "Karate Kid" not many people know "10 year old kid who becomes karate master" so the marketing idea is that people will go see it with Karate Kid branded on the film.
You know how many remakes of Yojimbo there are? Probably not... because the call them things like "Sword and the Sorcerer" "Fistfull of Dollars" and "Last Man Standing" instead.
It may suck, but it makes Hollywood money, which is why they do it.