With yet more years in education to get your teacher certificate, while your student debt accrues interest and grows, and which you will have to pay down in a career whose income potential has quite likely already peaked and is quite likely headed down over the next 20 years while you are competing for salary against people who don't need to pay down anywhere near as much debt and with additional qualifications that the schools don't care about at all.
Not to mention that there are magnet high schools that do very much like the perceived prestige of having PhD level instructors. Your typical public high school wouldn't care, of course.
That's not true everywhere. Where I live, there's nearly unlimited public school choice and school districts must reimburse each other at a state-mandated rate for transferred students. Especially in poorer districts, this rate is generally higher than the per-student funding the schools receive for in-district students. (I've heard it's up to $1500/year more.) Individual schools that can recruit students by offering specialized programs (languages, music, science, etc.) can increase their funding quite a bit.
Well, if you say so? Of course, it grossly oversimplifies reality. Job markets are complex; lack of success cannot always be traced back to bad choices. Also, you could level a very similar objection against health insurance. Personally, I'm not comfortable with that kind of senseless cut-throat capitalism.