there's a great planet money podcast that covers this

It turns out childcare is just a very labor intensive sector due to the amount of adults needed to watch the kids. This means that most of the costs for daycare are already labor:

In fast food, labor is 25% of the total costs. Estimates for day care - it's, like, more than 70% of total costs.

Given that labor is already a disproportionate amount of the costs, raising salaries a little has a large effect on the increased cost that parents would have to pay.

For some real world numbers, daycare is around 1.3k/month for me. The school does 3 kids to 1 teacher, so that's a max of 3.9k/month of income per 1 teachers salary. So already, in an ideal world where 100% of what parents pay go directly to the teacher, the max they'd make is 48k/year.

Factor in things like renting the facilities, utility costs, administrative and security staff, taxes, etc. and you quickly start to see why even though it seems like parents are paying a lot, there's just not that much money to go around.