There are thousands of possible markets for a freelancer, and if you;re any good, you'll have far more markets than you have time to write for. Many freelance writers break the $100,000 word mark.
The problem is that even very few who "freelance" really know much about the business. It's the easiest form of writing there is to earn a good, steady, full-time living.
It is easier if you're a recognized expert in some high demand field, but anyone with talent can find areas of freelancing that are high-paying and wide open, if you have the talent and the work ethic to fill them.
Darned near every business out there, from small start up to giant corporation, is a freelance opportunity, and big business pays big bucks. Magazines, too, are freelance opportunities, and I've known several writers who earned a steady $60,000 per year just writing for magazines.
I stopped freelancing because I wanted to spend my older years dedicated to fiction, not because money is hard to come by in freelancing.
But there's a lot of money to be made in fiction, as well. Freelance writing or fiction writing, it's about having the talent, learning the business, and having the work ethic.