I'm thinking, actually one of the main reasons for this is because autistic girls are easier than autistic boys. Autistic girls experience the same things inside and have a hard time dealing with things, can't figure out why they feel different than their peers, etc. Maybe seem OVERLY shy, OVERLY quiet, etc. But these things are easier for parents and teachers to deal with.
2 boys and 1 girl.
The girl won't join in things unless an adult leads her to join. She's super quiet and doesn't talk to anyone except a handful of people (talks to mom, dad, grandparents, siblings. doesn't talk to aunts, uncles, friends of family, most peers). She feels left out because her 1 friend sat with someone else, and though she could sit with them, does not because she doesn't know the other person. Has a harder time understanding and learning, but mimics her older sister, and probably spend most the evenings working on homework. It takes her the entire evening because of distractions, difficulty understanding what she's supposed to be doing, etc.
I could go on, but the point I'm making is that these things may be difficult for the girl, but it doesn't make life more difficult for anyone else.
The 2 boys are hard. Calls from teachers almost daily. Hits siblings and won't share - just grabs things from siblings or others. Won't listen, therefore won't behave as told. Non-stop busy with no limits they understand. Makes noises at inappropriate times. Again, I could go on and on.
Point being that these things interfere with the adults life and time having to supervise and it IS difficult for everyone around.
So, yes, it's more obvious in boys, but also it's just harder and the parents and teachers just realize they don't know what to do and need help and seek help from professionals. With girls, it's "aww, poor thing, you can see she's struggling." With boys, it's "HELP! I can't deal with this."