The same reason that ad usually show women doing the dishes and men coming home from work, the same reason that boys and men’s clothes are practical and comfortable, while girl’s and women’s clothes are designed to make them look more feminine, and comfort is secondary (pockets, anybody?).
It’s the same reason that boy’s toys are space craft, and trucks, and all sorts of thing that men do as hobbies or jobs outside the house, while girl’s toys are ovens and vacuum cleaners and baby carriages, and childcare stuff. When somebody actually makes anything like a boy’s toy for girls it is made in pink, so that everybody understands that it’s not to be taken seriously.
It’s why when a girl behaves like a boy, she’s a “tomboy”, and it’s “cute”. When a boy behaves like a girl, he’s a “sissy” and will be bullied, and he’ll be lucky if he isn’t also punished by his parents. A woman wearing a man’s clothes is OK, while a man wearing a woman’s clothes is a cross-dresser, and it’s considered pathological.
It’s the same reason that men’s Halloween costumes cover a wide range of characters and themes, while women’s are “Sexy ______”. The same reason that men think that it’s OK to send unsolicited photos of their private parts to women they barely know. It’s the same reason that women go out in groups and protect each other’s drinks at bars. It’s the same reason that it’s still an effective defense for a rapist to talk about the sexual activity of his victim, or the clothes she was wearing.
It’s the same reason that a woman in a uniform at a hospital is assumed to be a nurse, and a man is assumed to be a doctor.
It’s same reason that women are underrepresented in politics, the highest paying and most powerful jobs, etc.
It’s because we are still, at heart, a misogynistic patriarchal society which assumes that a women’s identity is determined by her husband. It is so deeply ingrained that most women buy into it as well.