* most mothers were still with their infants and very young children. The babies weren't being taken care at a daycare where you can have one staffer trying to deal with 4 crying newborns.
* mothers were doing active work, not work that requires sitting in one place, not work that requires long-term concentration, not work that requires being on someone else's schedule.
I have noticed it is no problem doing active work like cooking or the dishes or grocery shopping while bringing along an infant. But I cannot do computer work -- baby goes crazy from lack of stimulation.
Also, being on a schedule while trying to take care of a baby causes immense stress. What if you have a client meeting while baby is crying because he needs to be fed ... or is crying just due to lack of comfort and attention? Or sometimes (oftentimes) baby has a bad night and keeps you from sleeping, but you still need to be up and at work at a given time, instead of being able to nap when baby naps?
Both parents doing a schedule-bound, desk job while raising a newborn baby is not how we evolved to do things, and it's always going to be a source of stress and problems, even if you have "high quality" daycare available.