Because even with the best of wiping skills, there will still be some e. coli that migrates out of your **** continually, as well as the leaking of various other bodily fluids that are far more easily washed off of clothes than furniture. Actually, if you think about it, wearing shorts indoors is kind of gross - shedding all those skin cells all over the place. Outside is a different matter, because nature cleans and disinfects the outside world easily, but inside human-made structures, humans must put forth extra effort to clean and disinfect or pay the consequences of living in our own filth.

edit to add: I wanted to comment that the automatic editor makes it look like I said something worse than what I actually said. That was medically correct terminology for the hole that e. coli comes from.