[V99]

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.

[killjoywashere]

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

[bombcar]

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).