The most cherubic child starts to smell ripe after a few hours . My kids always threw up or had diarrhea at the conventions due to the water. Kids have sensitive tunnies. A lot of adults do too. Bathrooms frequently have a revolting stench at big conventions, Most poop stinks.
Add to to that all the stuff mentioned, IBS, UTI's. menstrual periods, eating nasty food that smells very foul when it exits the body, people who are into natural stuff and thus do not wear deodorant and on and on.
BTW, I used to freshen up my makeup and hair in the bathroom by using my compact. I would just stand somewhere in the bathroom to do it or if forced use the stall. I carry wet wipes. . I did get glared at but I did not care. No one is going to tell me i can't freshen my hair and makeup if I wish. I also have a torn off a piece of paper off the mirror to freshen up . Admittedly I was careful not to be seen doing it, but did it anyhow.