With respect have you ever ridden a New York or any major city’s subway during rush hour? Building a shell of some sort may be the only way to stay sane in such an inhuman environment.
Look at the picture of the subway car up there. Absolutely minimal comfort of the sort usually seen in a police holding cell (and in fact, the holding cell will probably have a toilet and drinking water) and obviously intended to jam the maximum number of people into the smallest possible volume which means that is exactly what happens.
“Human interaction” is just about the last thing most people want in such an environment.
