I'm not from the US. I have always looked weird at the idea to separate scouts based on gender. It makes sense in sports, not in scouts.

Well, a lot of Scouting is like sports. For example, boys tend to be stronger hikers, they can be more resilient than girls, they tend to like more logical activities while girls tend to like more creative activities.

If you had a scout troop with 9 boys and 1 girl, chances are the entire group will have to go on shorter hikes to accomodate the one girl. That's not a certainty, but it's a strong possibility. The physical and psychological differences between boys and girls are why Boy Scouts and Girl Scouts have remained separate for so long. People who think alike tend to band together better, and it creates a valuable sense of camaraderie and belonging. It makes sense, then, to separate the troops by gender.