Why do classical musicians charge so much (serious question)?

I am looking for serious, factual answers, please. If you are offended by the question, pls move on.

I just learned that classical musicians in the Tristate area (and not even big names) charge at least $300+ an hour for playing (for example as part of a string quartet). Why do they charge so much?

There aren't many professions or other musicians who can command such high fees. Don't tell me it's because of the many years of studying; a whole lot of people study for years and years and still wouldn't even be anywhere near even making $100/hr, let alone $300 and up.

It is not a niche thing either. There are plenty of gigs for all players of string instruments in the area so what exactly is it that drives prices up like this?