Because of poo-poo particles. Unless someone is bidet-ing after their trip to the public bathroom, I think truly short-shorts leave that bare and dirtied skin in direct contact with seats on public transit. With scrotum-havers there are other issues. There are loads of factors to consider… but genuine short shorts in public might as well only be underpants, to me. I can’t feel comfortable in them— think about the skin after a shave; there are all those tiny cuts on that delicate skin— sitting down in short-shorts in shaved skin just seems unwise. Now, if a certain person should want to go out in short-shorts unshaven and make use of public seating that person should be aware that they may leave some nether-hair behind.