When looking for rocket engine images, I noticed that the bottom of the stages are almost flat. That does not seem aerodynamic. I understand this is not a primary concern for upper stages (mainly operating into the vacuum of space) but it seems weird for the first stage.
Why are the bottom of the most (if not all) first stages flat and not more aerodynamically shaped?
Edit: given the answers and comments, I should highlight that the rear of a supersonic jet fighter seems more aerodynamic (contains only shap edges and engine exhaust, no flat surfaces perpendicular to airflow) and that at least one rocket first stage tried a design that seems more aerodynamic (the tampered bottom of the Ariane V)