Because human beings, like other primates, are hard-wired to expect a sexual division of labour.  Yes, in hunter-gatherer societies it's the males who went after the big game.  But even in those societies there are many roles that are sex-specific but have nothing to do with size or physical attributes.  The division of sex roles is often quite arbitrary.  With a few obvious exceptions, such as the hunting example you mentioned, it doesn’t really matter what role is assigned to which sex.  The important thing is that they're a binary opposition.  Think de Saussure, think Levi-Strauss.

I’d argue that the perverse attempt to steamroll over human nature is the cause of much mental illness today, particularly gender dysphoria and disorders of sexual attraction.