>>280066545It's because the girls are the main product, the music only sells because it's song by cute girls who are market no different than idols are, if they don't have a mixed media project where the seyuus themselves perform and having guys mixed in would obviously change how the dynamic of the group is perceived.
It goes without saying why you won't have boys in works published in Yuri Hime or all female cast from yuri adjacent magazines like Kirara, as it would defeat the point of those to start with. The others have different degrees of yuri pandering because they understand very well there is a overlap between those audiences.