We probably should. I bet it's easier to teach a STEM grad how to teach (in a professional training program after college) than to teach a professional 'teacher' how to do math.
My sister is a 3rd-grade teacher and I'm not sure I would be equipped to handle 30 children without lots of training. Teaching someone how to teach 3rd-grade math is not that difficult. Teaching someone to start building positive learning habits with 8-9-year-olds sounds incredibly complicated.
I might be reading into your post too much but it seems like you think STEM grads are superior to non-stem grads. I don't buy that. STEM grads have a specialty that is in demand in the labor market. It doesn't make STEM grads better then non-STEM grads.
You may be right about elementary school teachers, I'm thinking more about high school STEM teachers specifically. Although the grade school teachers in my city seem to struggle even with fractions [0].
Only one of my high school math teachers seemed to have any idea what math was about (not just regurgitating formulas, but deriving them and proving them from a set of axioms and only then using those formulas to solve free-form real-world problems).
My High School CS teacher was a geography teacher who just read one chapter ahead in the CS textbook and had no idea what he was talking about.
Being good at teaching seems kind of useless when you don't know anything to teach.
Well, the real question is marginal value - if we taught more software engineers, they might not bring in much income or create much GDP.
If you're asserting that teachers are as valuable as software engineers to society, then the question becomes: why doesn't society compensate them accordingly?