I get that they would be aerodynamic in-atmosphere, but so would cigar shaped or pyramidal shaped ones. Is is a rule of cool thing? Preferable to cite some Star Wars reference. Legend/Cannon doesn’t matter, I’ll accept what ever gives a better reason.
Edit: The Star Destroyers in question consist of the following list.
Acclamators, Venators, Imperial I & II, Victory, Super, Xyston (the planet killers, I think), and any others shown on film or in Legends books.