Probably because times change. Stories are only "timeless" in an emotional sense. Historical/period pieces are an exception, but only if they are nonfictional.
As far as comic books go, you have to revamp things. With advances in science and technology and an ever-changing social and political landscape, what may have held water as close as last year might not hold water today. Doesn't mean you have to change what makes a character "tick", but the situations that character finds him/herself in will change.
When it comes to revamping the story from the bottom up, who knows? I guess some people think that a classic tale gets stale over time. I can see some value in that, depending on the story. Cinderella, for example, is very backwards if you look at it from a modern perspective. Most women today probably don't want to wait around for Prince Charming to put the glass slipper on their foot and release them from a life of misery and servitude. Same goes for a lot of the old fairy tales with similar premises... the damsel in distress waiting around for the valiant knight to save them. On the other hand, a story like The Boy Who Cried Wolf doesn't need to be revamped. The moral of the story holds true as much today as it did when that story was first told.