Earlier quoted context omitted.
And then you can take another step and ask why professions dominated by women in general are less well paid than similar ones dominated by men, and why it is mostly women who reduce their work hours while the kids are small and why not more men do the same.
Because the jobs are less valuable, not due to who's doing them but due to the market (i.e. all of us) seeing them as less valuable. Teachers and nannies aren't paid a whole lot because they're easily replaceable, not because they're jobs dominated by women. Doctors used to be exclusively men. Have salaries gone down now that women make up a significant percent of the number of doctors?
I don't know any nannies, but it's not true that teachers are easily replaceable. Not good ones, anyway.
Good teachers need to show these skills:
* Classroom management (possibly in classes with disturbed kids)
* Curriculum knowledge (requires regular training)
* Social work skills
* Child psychology
* Understanding of various learning impediments
The fact that teachers are cheap is not evidence that they're easily replaced.