Because some guy's junk swinging past at eye level just makes a 7-year-old's day.
Because worrying constantly about people/friends dropping by enhances your life immessurably
Because clothes are such a Huge problem....

I grew up on a Haiti mission, and since the local Haitians typically owned only one set of clothes, on wash days, everyone went naked until their clothes dried, but nobody seemed to notice or care.

My European friends went nude on the beach and after I was married, since my husband also grew up on a Haiti mission, we both ran around nude when indoors, and our two children as well, except they were able to play in the fenced yard, nude, as well.
When they were about seven, they themselves started trying to cover up, so it was back to clothes for everyone.

Don't children have enough crap to deal with already? I'm sure that goes over real well when they discuss it with other non-nudist kids.