Why not rear-load like military aircraft and avoid that 90º turn? Then you could have a hull-shaped hopper to drop the bags in, so they are tetris'd together in the right geometry before you get to the ramp. Roll the hopper in, have a pez-despenser style pusher so as you withdraw the hopper, the bags are left behind in the aircraft?

Step one would be "get Boeing or Airbus to design, build, certify, and sell thousands of a significantly different airplane model".

That's a big hurdle for an airline to start climbing now to someday maybe start saving money on bag loading in many years.

surely someone thought of this before though. There must be a "why not" answer already.

It's expensive and difficult to put a large door on the front or back of a plane, resulting in inefficiencies or maintenance headaches.

The planes that DO have them are almost always military planes not used for other things, or very large jumbos (search 747 cargo or C-5 galaxy).

Almost all other planes are optimized for passengers (the cargo space in even a largish jet may be too short to stand up in, for example).