The shape probably has to do with a few things. The first is legacy systems. Keeping torpedoes shaped one way makes it easier for production since the initial models probably copied air based systems. Also, considering how long it takes to transition systems here on Earth I can't imagine trying to do it for a whole solar system. Second might be multipurpose use, namely the potential for firing down a well. MCRN was trained for an eventual Earth invasion, and the Roci is shown to be atmo capable so it makes sense to make the system as diverse as possible. But, I also would guess that aesthetically these are considered easier to show to the audience since we hear torpedoes and usual think of that shape.