Because employment status changes, family expenses increase, children are born, cars break, people get sick. Yes, In a happy marriage, you should be looking to be useful and supportive to your partner, but again, hang out on r/marriage and you'll see plenty of people who are not. So they fight over who has to pay and whether one partner should have to "lend money" to the other when they unexpectedly needed new tires.

I became the stay at home parent for 6 years. Thankfully I have a loving, supportive partner who values my self care more than I do. That's not the case for everyone.