Before children, the actual volume of housework is less. They are both out at work all day, so no mess is being created in the house. They may also go out together several evenings a week. There is usually spare money for convenience food, take-aways, and eating out more.
The load may be equal, but for both of them it is just far less of a load.
After children, one parent is home all day, making mess, e.g. lunch at home (which takes extra planning, i.e. making sure there is something suitable to eat in the house) is another round of tidying the kitchen and loading the dishwasher; even if it is minimal, it all adds up. Obviously there is masses more laundry. From a few weeks old there are toys strewn around.
As soon as baby is starting on solids, the food gets everywhere, more mess. Plus usually money is tighter, so more meals are cooked at home.
Then the 'bedtime routine' takes up what used to be free time in the evening.
Plus a whole new level of mental load comes with a child.
Even if the DH tries to step up, the shear volume of extra work and thought is more than they are prepared to take on, especially if they are working full time and the woman goes part time.
Plus the DH often becomes acutely aware of the financial responsibility (NAMALT), so is putting in extra hours at work, or working more intensely to try for promotion - there is no scope for a half-arsed performance level at work.
Add to that, many men don't actually want to be a parent. They may agree to a baby because otherwise the woman will leave. They may quite like the idea, but are utterly clueless about what will be involved and just assume that the woman will do it all because she is the one who 'really' wants the baby. They act as if the baby is her new time-consuming hobby, something she chose and should enjoy, but it is up to her to deal with it, not him.
Or, they want to "be a father" as a status, but want a woman to do the actual work of parenting.