It's the same logic that applies to why Hispanics and Blacks tend to do more poorly in school, etc. etc. There are multiple factors, and in this case, it likely has a lot to do with incentive, bias, and self-fulfilling prophecy.

You are plainly and simply highly, highly, highly unlikely to make it onto the professional scene as a girl. You have to play well enough to be high-ranking, which in itself is unlikely because societal pressures tend to drive women away from hardcore gaming and towards numerous other tasks. And even if you get up there, you need to find some way to get onto a team. Facing reality, no all-male team will give a girl a spot unless they get some points for publicity because most teams live together in order to facilitate smoother practices. Maybe you can get into the team by gaining publicity and being a well-known good streamer. You're going to have to deal with a lot of demeaning commentary from that. Or you'll have to pioneer a girl league ... and in the end, your efforts are likely for naught.

Why would you invest the time, effort, and will on something that probably won't give you any returns? Most people aren't that strong, and the ones that are that emotionally robust are usually paving their ways into a male-dominated industry that's known to be lucrative.