most girls in modern america are still bound by limitations imposed by society. most of the time these limitations are largely internal and it's very easy not to pay attention to them. as a guy you generally grow up with the mindset that you can do what you want and that what you want is important. for many girls it's hard to realize that they are the main character in their own story. you've probably witnessed this sort of self imposed alienation yourself: girls who say "sorry" too much or girls who say "this is going to sound stupid but...", you can chalk it up to 'women's natural tendency to be submissive' or some other sexist explanation but you've undoubtedly encountered this sort of behavior.
many girls simply don't have hobbies the same way that boys do, and many of them are in fact afraid to try new things. i imagine that over time this will change but there are a lot of aspects of modern american society which reinforce this status quo. the things women are most regularly celebrated for, the lack of important female characters in media, the way that most men treat women. it's very difficult to break into something where you don't feel like you naturally belong.
i don't think the guys who play tabletop are to blame but at the same time they are very accustomed to playing without girls and making these games coed can change things in significant ways. people are generally always wary of change and since tabletop is an often maligned niche community it's easy and justifiable for players to be defensive about it.
tl;dr i once ate an entire rotisserie chicken in the grocery store parking lot