If you already have an oval seat tube, it may be simpler to engineer an oval post for it than a cylindrical one, and bike companies are building bikes to sell new, not to be useful to the second owner because that makes the zero dollars. So, the question isn't necessarily whether there's any real aerodynamic advantage for the mythical Average Rider, but rather whether there's any advantage to the bike company in spec'ing a round seatpost that is unarguably less aerodynamic.