Through high school includes high school. It's also not just the injury itself that pulls them out of the game, it's the 6 months or a year of lost practice from rest, surgery, or rehab and the associated lost rhythm. Pitching is indeed a many year journey. So, while I'd like to take your money, since we are both pulling numbers out of rear ends, I'll have to settle for agreeing with myself. And yes, many stop because pitching is an insane amount of work. My 2 daughter's would tell you, despite that, it's way more fun than other positions because the ball is in your hand every single out and the hard work is a fun part of the journey.Would bet many, many thousands of dollars that the percentage of girls who stop pitching because of serious overuse injuries before they reach high school is nowhere near 30%. That's a massive number, and it certainly doesn't align with anything I've seen or heard about.
Most of them stop pitching because pitching is hard and it takes an insane amount of work to just be adequate and it's way more fun to play other positions.