Because they have their own competitive category and junior period is already 6 years allowing for variation of growth and motor skills development. The 15 yo entry in both men and women doesn't mean they all have to start at 15, they have a 4 year window to chose their peak. Sure, some women started seniors at 15 because they were ready to compete at that level, but not all of them, and the same happens in men. It is long enough and averaged. If you start at 17, then to compensate, the window needs to widen upward, allowing them to stay in juniors till 21, because if they only have two years for transition an injury at the wrong time is far more consequential. Then you get junior division encompassing 13 to 21.So why not allow 12-years girls in juniors or even younger? Some of them already have quads, but who knows what will happen in a year or two.
I would also want to note that sports all have specificity. I remember seeing male gymnasts in Olympics this summer, and they make pair male skaters look like willowy adolescents. Gymnastics routines are shorter and burstier. Skating relies on endurance component of fitness more. Body type for men for single skating is less representative of an average adult men than for women. But, again and again, it is the women who draw scrutiny.