It is unfair if one parent has a to do list (mental or written) with 100 things on it and one parent has a to do list with 3 things on it. Or in some cases, no to do list.
It is unfair if one parent is sitting in front of the tv researching birthday party venues, texting mutual friends to organise social stuff, ordering hockey socks, shopping around for insurance, amending the food shop - and the other one is scrolling Instagram because none of that stuff is their problem.
It is unfair if one parent spends their weekends meal planning and cooking and figuring out when everyone needs to be where - and the other one just does what they are asked with zero thinking.
Even if both parents do equal tasks (which is rare), if only one of them is thinking and remembering and planning what needs to be done, and the other one only does what they are asked and reminded multiple times, then the load is not equal. Because one parent’s headspace is full of what needs doing next and the other’s is empty for them to think about whatever is important to them.
Managers at work get paid more, due to the extra responsibility. And they get expected to do less of the actual work, because managing (planning, coordinating, chasing up) everyone else is seen as a big job in itself. If one parent is the “manager” but still expected to do half the jobs, or more, that is unfair.
I don’t know how else to explain it 🤷♀️ If you don’t understand the mental load then you either live a very simple life (probably without kids or with one small child or much older kids) or you’re leaving it all to your partner.