Solo freelancers (to whom OP is referring) do not bill 40 hours a week unless they're working a
ton or incredibly dishonest. A good portion of your time is spent marketing, doing unpaid pseudo-work-pseudo-marketing like GitHub side projects, billing clients, chasing unpaid bills, tweaking your website (marketing), networking (marketing), etc. Not to mention most freelancers become freelancers so they can avoid the 40 hour work week! When I freelanced full-time I tried not to work more than 30 hours a week all in. I usually billed about 15 hours a week but only because I had a few big clients and didn't need to do much marketing as they sustained me for the 2-3 years I was doing it.
A $150/hr freelancer who is busy for the year could reasonably expect to pull in about $100k gross (before any taxes, etc) assuming they take a few weeks off each year and bill about half their time or a little less. More if they automate a lot of marketing, have a local VA or someone doing the client phone calls, etc, allowing them to bill more. So the very successful ones are probably in the $150-175k gross revenue range before their taxes or paying anyone helping them.
I'm not saying @sidlls's implication that $150/hr is poorly paid isn't idiotic, because it absolutely is. Just pointing out that $300k will not happen strictly from trading 1 hour of work for $150 at any scale reasonable for a 1-person freelance shop.