I don't want to give away the ending, but I think it had something to do with the idea that they wanted to use fear to control her. I picked up on the idea that they are from a society where fear works on a level that is stronger and more ingrained. The idea is more about what's an easier thing to swallow when it comes to evi. It's easier to picture an inhuman monster is responsible for your woes rather than a fellow human being. Most horror explores this theme, but I thought this was why the premise did work especially well here.

How is "There are mutant monsters and a post-apocalyptic wasteland outside" scarier than "The government will take you away and maybe kill you."?

Seriously, the main reason the girl wanted to leave and the main reason she got angry at them was because they had her living in a grimy little bunker and she longed to be in a nice bright clean house.

Why concoct the grimy bunker & mutant-infested wasteland scenario when she might have been less prone to leave if they built her a nice bright place to live in and honestly told her government agents would haul her off if she left?

The only reason was to set up the surprise twist for the audience.

Premise was dumb, but not dumber than many horror shorts.

"An entire movie of muffled breathing masks!"