There are between 3-4 million developers in the US. Assuming 30,000 of these $300k jobs (not likely), that's 1%.
> I would argue that the skill level required in its employees for a startup to not go out of business is at least as high as that at these top companies.
Most startup jobs are just another React or Rails or Node job. You're not going to get a $300k job doing that kind of work.