What I would say, if I 'm being brutally honest, is that most of young children's life experience happens between the hours of say 7am (or whenever they wake up) and 7/8 pm. The daytime hours are when they are absorbing the world - learning, exploring, socialising, communicating - everything basically. They are like sponges and the rate of learning and development in this phase is astonishing.
Now children will obviously learn and absorb in the mother's / father's absence. But some women will always believe that they want to be influential and directive in that 'absorbing.' They want to be a part of their child's rapid development in their most active hours. They don't want to leave this crucial time to a third party, basically. For some women, not being with your baby / young child can feel as if it goes against every instinct in you body and it's overwhelming. Trying to deny that instinct can make you depressed and / or physically ill.
I'm sorry if this offends anyone and I'm not suggesting for one minute that children can't learn in other settings, but fundamentally, this is why some women feel it's vital for them to be the one with their babies / children in the daytimes and in the greatest. 'window of opportunity" if you can call it that. It's not just 'childcare.' It's so much more than that and it's something they don't want to outsource. It's as simple as that.
Hiever, I totally understand that many women get so bored being around children that they wouldn't stimulate them anyway. That's fine and if that's the case, far better to admit it and seek an alternative, rather than plod on and make yourself ill.
And I also recognise that many women who would love to SAH don't have the option to. This is the reality of the cost of living today. But they are no less great mums and they are doing the best they can.
Finally, no mum is perfect. Children just accept their parents for who they are - working all hours, part/time or none- and hopefully its enough. We can all only do our best according to our personality types and the financial context we are in.