Just a question that's been bugging me and my google fue is failing me. Why are all shotgun shells rimmed and not rimless?(aside from "shotshells")

Because the chamber is deeper than the case length, to allow the crimp fold (or rolled in crimp on slugs) to extend out ward to let the shot (or slug) move forward without being hindered by the plastic case folding out.

Otherwise there would have to be a shoulder in the chamber to prevent the "rimless" shotgun shell you speak of from going too deep into the chamber and out of reach of the firing pin.  Then there would be the pressure spike from the plastic shell of the crimp folding out over that shoulder, and reducing the bore dimensions for the shot or slug to pass through to get into the barrel from the chamber...past the unfolded or unrolled crimp.

Damn, now THATS an answer!