Ok, but imagine you're there, cracking on with all the stuff you're doing to manage your own life, and the home you share with your partner, and the car you share with your partner, and the kids who are also your partner's kids.
And while you're going about all the adult life stuff - all the mortgage refinancing, chasing up medical referrals for the DC, getting all the forms in that school are constantly asking for, getting the boiler repaired, booking the kids into the dental hygienist, getting the car MOTd - your partner is snoozing on the sofa, or out playing golf, or standing in front of the fridge absentmindedly snacking on the food you've bought for the meals you'd planned for the week and the kids' packed lunches.
In this scenario, what is it that you are doing, that your partner (whose kids these also are, whose home this also is) is not doing? That's the mental load.
It is being an adult - which feels busy but do-able when all the adults in the family unit are doing it, but feels emotionally very burdensome when you've got another perfectly able-bodied, able-minded adult benefitting from all this adulting, who is just sat there watching you do it without pitching in, or even bothering to know what any of the things are that need doing, and by when.
To put it another way, imagine if you could just come home from work and just not have to think about being an adult with responsibility for your home environment or the day to day business of raising your kids.
How much more energy would you have to put into your career, your hobbies, your social life? If you just opted out of even knowing about 90% of the boring but necessary shite of daily life?