You could have one on each side, and adjustment screw equipped dropouts do just that. By having at least one it provides a reasonably repeatable wheel placement and its easier to pivot off one side than just have both sides float to place the wheel.

The horizontal dropout design is a legacy from single ratio bikes and then the Campagnolo "Cambio" shifting systems. It was expected essentially. There is also the truth that making a vertical dropout frame requires a bit more effort, things must be more exact. Plenty of frames of "top tier" quality require one of the adjustment screws to be a bit more set back to have the rear wheel sit straight.