Because if they were competing with a boy who had that girl's exceptional talent, that girl would not be winning. Yet, those parents would write that off as that girl not being good enough, or training hard enough in comparison to the boys she is competing against. And they would not even consider the impact that this would have on that girl. All because some parents will declare, it is just a bit of fun, or that 'girls can compete against boys at x age'.
Yes this is really often overlooked. Girls who are competitive in boys teams above the age of puberty are exceptional at their sport.
They often continue to play in those teams because there are not many opportunities in teams of their own sex and they do it for development reasons. They are there on merit and because they are prepared to take the risk that is posed by the power and strength of the boys on the team.
In my daughters sport, which is even more under represented for girls than for boys than football, other than county fixtures she doesn't really get all that competition/ development any more in playing in girls teams. She won player of the season in a local women's team when she was only 13. So she does play in boys and lower level mens cricket teams because she can cope at that level, whereas some of the boys in her boys mixed team aren't good enough at the sport yet to do that. But if any of those boys went and joined the girls team for the same age group it would be a massive risk to the girls on the team.
A boy of her age at county level wouldn't have any interest in either her boys or men's teams because they would be too good, they play in higher leagues because they have both the talent/ technique and strength/speed from being male/ going through male puberty.
I always think that the fact that Serena Williams says she would lose 6-0 6-0 to Andy Murray illustrates the point well. Serena is an amazing athlete but would not have a chance against ranked males. https://www.facebook.com/watch/?v=1606459836413781