Because they don't scale.
This is like asking why JK Rowling is a billionaire but your neighbor who only sold 10 children's books is not.
Teachers and Scientists who scale are billionaires[1][2] or centimillionaires [3]
1. https://www.cnbc.com/2019/07/30/indias-newest-billionaire-is...
2. James Watson, Ronda Stryker, Gordon Moore
3. Stephen Hawking, Richard Dawkins, Jane Goodall, Noam Chomsky