It's a slow and gradual creep.....
On mat leave women become the default parent.
Then work part time, managing the home and family (being there for the kids, extortionate nursery fees, next pregnancy etc).
As kids get bigger so does the responsibility. You may be back working full time but the position of default parent is now well established.
The men don't see (intentionally/unintentionally) what needs doing.
School runs, dentist, doctors, parties, play dates, uniform, clubs, hair cuts etc
HAVING TO TELL THEM, REMIND THEM, JUST ADDS TO THE MENTAL LOAD! We need them to see something needs doing and just do it!
We don't need "just tell me what to do". Use "adult" as a verb as well as a noun!!
Kids hit teens, we hit menopause, the inequity hits home. We realise the extent to which careers, hobbies, lives have been put on hold.
It's why in mid life many women blow up and men look perplexed.
Then throw some older generation family into the mix to add to the joy.
My DD has discussed it with me and said to hell with that! As a Dr her career will be important, equity all the way for her with compromise on both sides when needed.