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human
ELI5: why are Girl Scouts and Boy Scouts gender segregated?

Edit: in the US. It seems that other countries aren't so strict in segregation in similar organizations.
gpt
Girl Guides. 

But they aren't, at least not any more. Girls can and do freely join Scouting bodies that were previously devoted exclusively to boys.

The reason for the segregation was because boy scouts was created very early in the 1900's by Robert Baden-Powell as a very traditional organization in a time when women had a different expected homemaker role. So the boys got to do cool stuff like woodland survival and the girls concentrated a bit more in in-house stuff like sewing and cooking.

Although this has dramatically changed, you still see some of this in the selected activities where the Scouts do more adventuresome stuff than the Guides. So more adventuresome non-girly-girls that have a keener interest in the outdoor world have a tendency to go there.