I'm sure there are other factors too, but this is a big one for sure.

Just looking at my family, both my parents had a stay at home mom and 3 siblings. Me and all my cousins have at most 1 sibling, with both our parents working but we always had two grandmas that could watch us if needed.

Had I kept the same timetable as my parents, my hypothetical kids would have had not just both parents working full time, but all grandparents too!