Most Helpful Girl
We as a species are highly judgmental and selective in nature so those who might have more physical flaws than others might be less willing to let everybody see the entirety of their physical form.
It gets nippy outside a lot, especially for me. Also, while having perky gals has its obvious cosmetic benefits, I don't necessarily want them free as birds when I'm out on my evening jog.1
Most Helpful Guy
A lot of women shave their downstairs; it'll be unsanitary for women not to have anything protecting their body from dirt. (Underwear) - the bra is supposed to keep the breasts from moving too much while walking or to keep them from hurting by having no support. Men wear boxers most of the time to keep their junk in place and to keep their penises clean from dirt and other gross stuff.
We live in houses that mainly protect us from the cold or from the heat, so when we step outside our bodies aren't really used to it. (Unless you're outside often) - the rest of the clothes is to keep warmth or to keep the sun from burning skin.1