Because when you leave work, you generally don't have to keep doing it when you get home. Being a stay at home parent tends to be from eyes open to eyes closed kind of gig. There's a lot of planning that goes into every day, and some of it needs to be planned ahead of time. Things like dinners, for example. Or the doctors appointments someone else mentioned. Or that laundry needs to get done, dishes out away, shit I still need to go shopping for food for the next week, and while I'm out I need to fill this prescription, and after that I need to make sure I'm on time to pick the kids up from school. And once we all get home I need to make sure that they do any assigned homework or practice their instrument. And FUCK I FORGOT TO PULL MEAT OUT OF THE FREEZER.

Being the stay at home parent is a LOT of work... And it never ends. Parents don't get weekends off or union mandated lunch breaks.

And when the other half gets home from work, a lot of times the expectation is that they won't have to do much at home.

A lot of times going to work means you get to focus on something else and don't have to make all the decisions. The mental load gets shifted. You're getting told what to do rather than having to plan it all out. It's not always the case, but I'd argue that the majority of times it is.

And that's one of the things that can be difficult about being a parent and in a relationship. Making sure you're doing what you can to help lighten the load on your other half. And hell .. I'd argue that should be the case regardless of your parental status. Always be trying to make life easier for each other. Don't ever let it be one way.