A witch’s time is incredibly valuable. Every minute she’s not casting spells or performing rituals or experimenting with new magic is wasted.
So men do everything else. Not just the traditional “man’s work” chores like putting up the shelves, but also the traditional “women’s work” chores like cooking and cleaning, and the specialized work like raising the newts and harvesting their eyes, and dealing with the world outside the coven, and even jobs that we’d consider prestigious like engineering that in their world are still not worthy of a witch’s time.
It’s not about keeping the men down; in fact, “everyone knows” that men are better at cooking and cleaning, as well as math and singing, That isn’t actually true (any more than it’s true that women are better at sweeping and men are better at doctoring in our world), but everyone believes it any way (just as people did in our world until the last century or so).
The one thing the men can’t do is teach new witches, which is a very time-intensive task. Schooling can only do so much; magic is highly personal, and much of it has to be passed on directly from mother to daughter. If you have four daughters, that quadruples the time you have to spend teaching them, which is time as you have to take away from actual witching. (What happens if a mother dies? Her nearest female relative with no daughters is expected to adopt and raise the girl instead of having one of her own.)
But if you have one daughter and three sons, that’s three extra people that can be useful to the coven without needing to be trained by a witch, so you’re wasting a lot less time.
Why couldn’t there just be women who aren’t witches, so they can divide the labor between men and mundane women? Well, you could have a daughter and not raise her to be a witch, but it would be considered horribly uncouth. People would think you’re being unconscionably cruel to your daughter, not to mention depriving the community of a witch.
Maybe there are some cultures that did develop that way, but every major civilization today traces its descent to a handful of cultures (like the Greek, Hebrew, Indus Valley, and Yellow River cultures in our world) that don’t, so as far as everyone is concerned, this is the way things have always been. The different major civilizations can be different in major ways (e.g., westerners are polyandrous, centralians encourage male homosexuality, easters expect women to be serially monogamous and men compete to be a woman’s last baby-daddy, the one who gets to stay with her through menopause and into eventual retirement), but they all have no place for non-witch women in their sexual politics. The ones who did are thought of like the Philistines or Barbarians of our world.