In many respects, it's "modern" parenting behavior.

Example, if you let a baby cry at night it will usually stop crying after a few weeks.

Once kids hit ~7 years old it used to be normal to let them wander around the house and neighborhood unsupervised. In most unwesternized countries it still is.

That's how the average family has 6+ kids in some countries. They're only actively caring for a few at a time.

Taking such a relaxed view to parenting in the west would get your kids taken away

letting babies sleep alone is a modern parenting behavior. kids mostly used to sleep together with their parents before.

aside from that, older kids took care of younger kids, neighborhood groups took care of children together, older relatives who could no longer do hard work supervised the children...

And women were typically in or around the home.

Not a historian, but I'm guessing the primary reason is because for much of human history, two-parent families had one parent dedicated to raising children.

Also some other reasons:

1. Probably there was more general family help with children - grandparents/town/etc was more involved.

2. For kids that were a bit older, they were effectively part of the workforce - apprenticeships, farmhands, etc.

3. Probably, it was more safe to let kids wander around in many cases - cities are particularly complicated, what with streets full of cars and just generally unsafe areas for kids. In a small town, or a family farm, you could let toddlers run around a lot more.

manual labor like field work, house work, baking, tailoring, keeping a shop, etc are things you can do while watching kids. you take the kids to work, and let them play alongside you. when they are old enough they can help, or pretend to help.

if your work requires focus like writing or programming, then kids are a disruption and you can't do it.

also once factory work was developed, schools for everyone were built too, those developments happened at around the same time.

Another modern effect is that a lot of people don't live around extended family anymore which ends up affecting childcare options, especially with daycares in the state they are.