The same reason they don't wear bras. They think showing more skin, and "sex appeal" shudder will make them more attractive to the opposite sex. They have never heard the "less is more" idea that if you leave something to the imagination it is more sexy that baring it all.  Some men see boobs and think mmm, boobs, while overlooking the ham attached to those boobs. Also hams seem to have some obsession with sex, talking about sex, and fetishes, where humans keep that private.