Square Rockets?

I do not spend much time wondering about the design of military rockets. But I do find myself utterly perplexed as to why they make these things with a circular external cross-section.

If you are not bothered about whether your rocket is going to be trackable by radar, or even if you would like your rocket to be trackable by radar, then obviously a round cross-section makes a lot of sense. Round shapes are nice and visible to radar. And you get the maximum internal area, and avoid the problem that corners are particularly vulnerable to internal pressure. So yes, I can well see why the rockets which we send up to launch satellites, or to replenish the larders of the international space station, would have a round cross-section.

Conversely, if you do not want your rocket to be shot down by an anti-missile missile, you might be better off if the external shape of the rocket consists entirely of flat surfaces. That is how our modern fighter planes are made. They look a bit weird, as if designed by origami enthusiasts with a stack of black cardboard. But they have a much, much, smaller radar profile. So why not make your rocket out of square hollow sections, with a pyramid shape at the nose?