Because they are immature, lack courtesy and have no regard for others.
I see these groups from time to time. They come in, make a lot of noise, talk loudly about the guys-or not-, pose, talk on their cell phones ("hey, guess where I am? Yeah, that's right. You should see it. Oooh you are so funny!"), rattle the booth doors, go in and out of empty booths, and generally act like tourists.
Usually the clerks run them out if they are making a nuisance of themselves in other parts of the store also.
Most times, some of them come back individually, sometimes even in the same hour. Then someone invariably sidles up to them and asks them why they aren't still with all their little friends. I've seen a couple of the older guys go up and loudly ask them if they came back to give a blowjob or their ass fucked. Or if they were talking on a cell phone, ask them if they came back because they needed to make another phone call. That usually runs them off.