Even though I disagree with the premise that world doesn't need any more STEM, that doesn't relate to my question. I'm still interested in how to effectively teach stubbornness regardless.

A lot of stubbornness seems to blossom when a kid (or adult) realizes that there's something to understand beyond the next assignment. If you're working for the external rewards of 10 points/10 dollars/a marshmallow, once you've got it, you're done. If you are working to understand something cool, you have a lot more work to do -- and then you find something else cool, and then something else, and suddenly you're a stubborn researcher!

Many students I've met in high school or college don't even realize that there are cool things to understand in math. They think math is just a collection of rules. Once the rules are mastered/test is over, done. What's there to understand? And hence why be stubborn? What is there to be stubborn about?

A healthy disregard for authority is also important in stubbornness, and hard to teach in school (as it's against the self-interest of the harried teacher who needs a "well-controlled" classroom).