With working from home getting popular, even fewer people want to be a teacher. 5 days a week starting at 8am. Not going to happen.
The summers off are basically the only notable perk, too. At least in my state. Healthcare’s expensive and bad and gets worse every year. Pay’s bad. Work environment is usually bad. Retirement’s ok-ish but doesn’t close the gap on the poor pay.
Instead of one teacher teaching a class/subject (like math) 5 days a week, two teachers will rotate, one teaching math 3 days per week, and the other teaching 2 days per week.
But then, that would require them to double their supply of teachers, which seems unlikely to happen.
I don't think teachers pay is all that low (at least in my state), when you factor in they only work 50% as many hours - on an hourly basis, teachers (except perhaps the brand new ones) make pretty good hourly rate, full benefits, and are usually able to retire a lot earlier than regular folks, and with a defined-benefit pension and full healthcare.
Make them work even less? No thanks.
Are you making a mistake to confuse hours spent teaching in class with working hours? Teachers have a lot of other duties outside the classroom, preparing the lesson plans and grading papers being the most obvious examples.