There's nothing empirical about that 90-degree angle. It just seems to be a decent compromise for controlling the bike, opening the chest and diaphragm, and getting power from the glutes and back muscles. And let me qualify this. It's for performance riding, road or MTB. For tootling on the bike path on a cruiser or comfort bike, that angle will be closed up as the rider sits more upright and power is less of a concern.

As for getting aerodynamic, that's all in your back angle and it's your choice to go as low or high as you want. It's a matter of simply changing the handlebar height and modifying the stem length slightly, to maintain that angle.

And stock stem lengths aren't selected on looks. They're based on assumptions on the average proportions of the average rider with average posture for a given frame size. Larger frames come with longer stems because taller riders tend to need more reach relative to the cockpit length. They also tend to use more saddle setback and slacker seat tube angles to balance the weight of their larger torsos.