Sure, mental load for just yourself is fine.
But for example, when we went on holiday, I didn't just do my packing. I did my packing, and the kids packing. I made sure that we had sunblock and arm-bands and seasickness pills. I booked the holiday, the flights, the hire car, made sure I had my license (more than once we arrived and I then had to do all the driving because, despite reminding, he hadn't brought his), the passports (and they were in date), had it all timetabled, prepped the kids so they had hand luggage with what they needed plus emergency stuff etc.
He packed his toothbrush and some clothes (which, I had generally been the one to make sure were washed in time).
And that was just for a week away. Scale that up to everything every day. Every school requirement, every birthday, every tax return, car insurance, meal.
That's the mental load that I had, and he, being a lazy, self-absorbed git, didn't.
He used to say about this stuff "it'll sort itself out" - because from his point of view it did, but of course, "it" didn't sort itself out. I sorted it out.