The Crystal Gems have also been fighting for a good chunk of their time on Earth both before and after the gem war and have been trying to piece together what little they can now that Rose is gone. On top of this they had to raise a child with the only human they feel they can trust, Greg. It isn't a very good excuse, but I can see why they just didn't really give much of a fuck about trying to understand human culture. Even when not fighting, out of the entirety of the Crystal gem's militia the only people to even have shown an interest in the planet is Rose, who started the movement, Peridot who enjoys our social media and technology, Amethyst who enjoys the scene of music and actually has a confirmed human friend other than Greg, and Steven, who is half human. Oh and Navy if you count the fact that she thought the planet was 'lovely', which is only a potential to care about our ways.
TL;DR the crystal gems have been protecting the earth for so long they probably didn't even think or care that they were using the barn that Greg told them they could use.