I would guess that it's a whole lot of reasons including the reasons you stated. I would also add a couple (and there's probably more): Round is structurally

I would guess that it's a whole lot of reasons including the reasons you stated. I would also add a couple (and there's probably more): Round is structurally strong (which, when it comes to aerospace engineering, is really just the opposite side of the "light weight" coin). Symmetry of shape and mass makes the stability and control equations a hell of a lot simpler to deal with, and a round shape makes it easier to be (nearly) symmetrical in more ways than with other shapes..