Why don't airlines load their passengers like freight?
Boarding takes a long time, true. There are more efficient strategies (.pdf) which have been proven to speed up boarding by between 20% and 50%, still they are not adopted. Why?

You need to load the baggage, too. Also the catering. And fuel. You need to inspect the aircraft, run through checklists and brief the new crew. This all takes up time, too. Airlines do not see a big saving by loading passengers faster, and then waiting for all other jobs to be completed.

Your radical idea of containerizing passenger transport would certainly speed up things, but it would cost a lot of payload.


The most efficient pressure vessel is a sphere, and the next efficient is a cylinder with spherical ends. Pressurizing a boxy container is structurally very inefficient.
Loading passengers on palettes first and sliding those palettes through a narrow opening takes time in itself. And the palettes have their own weight, their own floor, their own support structure for the baggage racks, their own cabeling for the entertainment system. They would, however, not need their independent pressurization, so their weight impact would be moderate.
A possible "flatbed" airliner which could attain the shortest boarding times has only half as much fuselage height (or less) to transmit bending loads along its length. See here for an example of buckling in a fuselage skin due to bending loads. Now consider that reducing height by 50% will need four times more cross section in the longerons to keep stiffness constant. That will make the fuselage much heavier.


In the end, airlines go with the solution which maximizes their profits. By using a hollow cylinder and let their "cargo" self-load, they can maximize their payload and get tolerable boarding times. Tolerable for them, that is. That better loading procedures could allow you to come to the airport later is of no interest to them. If they would see an advantage in shorter boarding times, there would be ample of opportunity by improving strategies, even with existing airplane designs.