It's not that it's hard for them to quit. If you've ever worked low wage jobs you'll notice turnover is quite a bit higher than high wage jobs.
The reason they aren't paid much is it usually low wage jobs are pretty easy to hire and train for. Which means there is a very large potential applicant pool.
I wouldn't consider teachers low wage. Their salary doesn't seem that high for college graduates but they usually get an enormous amount of time off, pretty good pensions and and great job security.
