I would say it's because boys are most often going to be stronger and more athletic than most girls. Therefore, having a boy on a girls' team would give that team an unfair advantage over a team that is ALL girls. If a girl is participating on a boys' team, that is not a concern. It is a biological fact that boys are stronger than girls.
A girl participating on a boys team is not an issue so long as she doesn't force anyone from participating by taking a spot that is limited by roster numbers. If the number is unlimited then it's moot. But if there is a roster limit then a boy is getting shorted and that is way wrong. Especially if a girls alternative is available.
There is no girls Little League baseball. There is no girls HS baseball. There is is no girls collegiate baseball. If a girl wants to play baseball, she needs to join a boys league.
I'd like you to point out some examples of girls taking roster spots from boys who ARE BETTER AT PLAYING BASEBALL.
Girls have softball at those levels. Boys don't have softball. So should boys try out for the girls softball teams or should they create their own softball teams? The same way girls can create their own baseball teams just as they have created girls opportunities for virtually every other sport.
Baseball and softball are different sports. If a boy wanted to play HS softball, I have no problem with him trying out for the team. If he's good enough, he should be put on the team.
You are one butthurt loser. Mo'ne Davis was selected to play on the "All Star" team from her league. That means she was better than just about every kid who played in the league. She wasn't given anyone's spot. She earned it.