my sister was a teacher for 20-odd years, then she went through a Ph.D. program in education theory, and now teaches teachers. So below is not based on "knowing one or two teachers", but on knowing how teachers are trained/certified and what professional standards are.

Most teachers work hard over the summer, it isn't a break. Pay isn't great.

And a certain mainstream political group puts significant effort into demonizing the profession and destroying the pay/benefits packages... which matters, because in public schools, the politicians hold the purse strings.

People in here seem to be using "most" to mean "people I know personally".

To add another data point, every teacher I know uses at least a third of their "break" for lesson planning. Usually more.