Who exactly is this “they” you are talking about? Are you referring to the avatar on the app profile? Because that has an option for male or female.
You said so yourself the demographic is mostly female, so almost all stories the MC will be female. Almost all user authors are female/fem-identifying so they will write from the fem-POV. There are a quite a few stories where you have the option to choose the gender of the MC, but this requires a lot of branching and coding from the author’s end, so they won’t always do it. Besides a story is a lot better when the POV is defined.
Almost all other media has only male leads, if you’re so “offended” by girls and women having something fem-dominated its your problem