Lass
I'm glad you have a set up that works so well for you. But I'm finding your incredulity that anyone would consider a less than 50-50 split a little wearing.
Asking why that's not the case is a good question, but as pages of this thread have pointed out the imbalance in domestic duties for most couples, I think it's been answered time and again.
Usually one person has to undertake the bulk of the work at home, and it's generally women.
I'm glad I did most of the child rearing, but, in hindsight, I do think I may have done it slightly differently.
However. many women do want to be the main caregiver. This is part of the reason why I am sometimes on the fence about gender being an entirely social construct. From a biological point of view, if you go back far enough I'm sure children were breastfed for years, rather than months. Which would make child-rearing a female biological imperative.
And although has this has evolved to not be necessary, I wonder if the instinct remains.
If this is the case, I can see why culturally we perhaps lean more towards the domestic duties being undertaken by women.
However, it is now understood that in most families both parents need to earn. Therefore the cultural expectations should begin to accommodate this better.