When you're at work your manager carries the mental load, they organise tasks, ensure you have the raw materials to do the job, (hopefully) ensures enough staff are available and evaluates your performance (that's very broad and general but you know what I mean).
You don't expect your manager to then jump in and start actually doing the tasks, because their job is to plan and organise. They might in extreme circumstances, but day to day, you'd get on with your job (without too much interference or direction).
Problem is, this doesn't translate to the mental load of home so the person carrying the mental load (and let's me honest 95% of the time that's the woman in heterosexual couples) not only plans and organises the tasks (arranging appointments, nursery show rounds, play dates, creating shopping lists, planning meals, keeping track of laundry, when specific jobs need doing etc.) and ensures that the raw materials for the job are available (doing/ordering the shopping, ordering birthday presents, buying new uniforms) but also then performs as a member of staff by undertaking most (if not all) of the tasks so then takes children to the appointments, makes dinner, puts in the laundry etc. and if they want support they have to ask for it, because the other person in the relationship doesn't see or recognise the work which has gone into all of the planning. It causes a huge imbalance in levels of stress and responsibility within relationships.
The mental load is a job in its own right, but it isn't acknowledged, clearly, by the tone of the OP.