Because no, they're not the same thing, if I'd intended them to be identical tags, I wouldn't have bothered creating the after_ tag in the first place. I mean, if you're gonna' alias after_incest to incest, you might as well alias after_rape to rape, and after_sex to sex, and so on and so forth, while you're at it. The whole point of the tag was to let people specifically search for, or specifically avoid, images where the incest is not currently happening in-image. The imminent_incest tag already exists for that purpose, as far as concerns images where the incest is ABOUT to happen, so there's no reason there shouldn't be an after_incest tag for when the incest already happened.
Updated by LemmyNiscuit