Because it’s not just the super heroes that don’t age, but people like J. Jonah Jameson and Pepper Potts and whomever else. And once you start trying to explain all of this stuff, you shine a big spotlight on a problem that isn’t bothering the vast majority of readers in an attempt to make it work for the small few that care–and your explanation probably won’t make most of them happy either. It’s a sucker’s game, and it moves the Marvel Universe much further away from the real world in a way that would be difficult to ignore.