This is practically my specialist subject. Where would you like me to start?
I totally agree with Dahlen that housework/running a household (my preferred term) is seen in a way by both sexes as something insignificant. After all, you don't do a degree in it, do you? It isn't paid. No great historical figure has been known for their ability to clean their house.
So it is seen as trivial - men often don't see it as being their role to do it, and women often do it (a lot of it) but feel like moaning cows if they bring it up a lot.
Of course this isn't fair. It needs a bit of planning, some skill, and effort. It doesn't bloody do itself. (for those thinking 'skill? really?' - yes, skill. I had and know others who had male partners who had simply never cleaned for themselves. Didn't know how to hoover or what to clean a bathroom with. If you go from mum's house to uni to shared houses or cohabitations where someone else does it - they never learn).
I think particularly when couples get together young, neither of them is exactly a whizz at household stuff. Keeping your room fairly tidy isn't the same as running a whole household with kids and childcare to cope with. So sometimes a huge inability/reluctance to do much at all on that front just isn't apparent. It is a situation that can creep up on you.
Kids. The amount of household chores, and how hard it is to fit them in, rockets after you have a baby. Women who have done chores before as routine suddenly have a lot more on their plate, and start to resent it more. Then there,s the whole maternity leave issue. mothers take 6 months or a year or whatever at home - it becomes their 'domain'. Working fathers very much start to see it as not their job.
And when women return to work, it is often part time, or they earn less.... so they become by default both workers and the ones responsible for doing all the jobs their busy, harder working (in his eyes) and higher earning husband won't do.
Just a few reasons!