Yeah, ... well. Just because it wasn't specifically said, doesn't mean that it was not that way. I don't know where you've been living until now, or maybe you are just too young to know.
It didn't NEED to be written down anywhere. It was a general understanding that women are incompetent for certain areas of expertise. It's culture, if you will. Role models. Like girls wear pink and boys wear blue. Women do the housework and raise the children while men are the breadwinners, that kind of stuff. Men wear trousers, women wear dresses. Women are considered incompetent for certain jobs by default even in this day and age in western countries.
To overcome this cultural heritage a concious effort has to be made until the general understanding has changed, this takes time and some getting used to, but there is no way around it. And it's still a looong way to go - there's ample proof of this in this thread alone.