I mean you essentially need to learn a pseudo language on top of math to be able to do that. The barrier to entry compared to writing pen and paper is much higher, and math is hard enough for a vast number of Americans.
The context here isn't the average American, but a high school student taking an AP test involving writing math -- where scoring well counts for the same credit as having passed the equivalent entry level course(s) in college.
Besides, math is already pseudo language on top of pseudo language. Learning notation is part of math, I'll concede learning to type an expression on a computer is a different notation than writing it with pen, but if you can't learn that, why learn math? Each domain does its own thing too and isn't even necessarily consistent. Guy Steele gave a great talk a few years ago on "Computer Science Metanotation", and that's just one area of math: https://www.youtube.com/watch?v=dCuZkaaou0Q (Edit: And to conclude here, learning to read and write with some notation tends to be easy, it's learning what the math actually means that is the hard part.)
You can say this, but you won't like the response. The response is probably "okay" and they just won't learn math. I don't think the world is better off when that happens. It's not a question of whether people are capable of doing this, it's a question of whether this is worthwhile.
