They could hire retired people from the private sector, but that would screw up the union dues.
I speak with first hand experience on this particular issue. Schools will hire retired people; however, one still must get a teaching certificate in order to teach. A person can teach in Georgia on a permit for three years, and that permit can be renewed if there is evidence of coursework toward certification. A certificate is equivalent to a master's degree. That represents a great deal of work for a retired person.
At any rate, I taught Embedded Computing (Computer Science) at the local high school for three years. I enjoyed working with the students, and all of mine were actually interested in the course. The trouble is, high schools systems are bound to state standards, which were clearly written by someone who didn't know anything at all about computers, programming languages, or electronics. The schools don't believe what I tell them or are unable to react because most assume that a PhD in Education knows more about computers than someone who made his living actually working projects for 47 years. (It is actually much more complicated than this.) Now get this: Computer Science courses are clumped together with sewing, culinary arts, and marketing rather than physics, math or engineering. I couldn't even have functional peers in a teaching team.
There is no way to teach difficult subjects in high school until the education system is sacked and rebuilt from scratch.
“They could hire retired people from the private sector”
DH is PhD in hard sciences but can’t teach school in TN without getting a special certificate. He does tutor some kids at church in Science and Math, and the stories they tell about their Neanderthal teachers are scary.