Another classic example of girls that are capable of escaping bears being unrepresented in tech. I kid, by seriously the cause of this isn't game devs being sexest against girls, it's (IMO) society being sexist in general. Boys (not men) will not play games that look like they may be made for girls because of what society might think of them. The fear of being bullied for enjoying anything pink or featuring women is very real for most boys growing up, especially (but not only) if the boy is already feeling confused and ashamed of what his sexuality might be. Since the mobile game market is dominated by children, having the main character be female can potentially scare away a large percentage of the market. Unless the game is already clearly "girly", then the risk just isn't worth it.
TL;DR: Boys playing girls = gay, girls playing boys = normal.