Serious freelancers who try to make a decent living out of it don't use those sites, for several reasons, some of which I'll go through.
1: Overall(note the word, overall), they cater to simple projects in oversaturated fields.
2: They get flooded by unscrupulous or simply cheap people who offer pay way below what is decent.
3: You get no way of building up a decent reputation.
4: As a combination of the above factors, you have to churn through lots of contracts constantly, increasing risk of burnout, failed contracts etc.
On the other hand, to make a decent living, both in pay and in the way of hours you work, you want to work in specialist niches, one contract at a time, maybe two overlapping at a pinch, if they don't interfere with each other(Starting the design phase of a new project as you're working on the testing/debugging/deployment phase of your previous one works ok usually, while starting a new project while in the development/coding phase of the previous one is usually not so good...)
You want to establish a good reputation and a wide contact network. And always make sure that you have a lawyer of your own go through the contracts, and the help of a good accountant. In fact, the more familiarity you get with your clients, the better, since you will get more leeway in case of sickness/family issues/issues beyond your control etc.
Avoiding the use of those websites, and working via agents instead, also gives you better options for negotiation(especially if you have the advise of a lawyer and/or an accountant, depending on the issues you need advise on), and can better structure your life. It also allows you to check out potential clients much more easily. Some of your contacts, or your contacts contacts, may know about some issues that have not made it into public records for example. Point in case, a contract was offered to my agent once, which he immediately blacklisted. Why? Because he checked up on some of the people running the company, and found major financial discrepancies, such as the company nominally running at a loss, CEO supposedly earning only 20k euro per year from that post and a total yearly income of 40k euro per year yet still owning a yacht worth about 2M euro etc.
I have freelanced for about 15 years now, and while I initially had to take risks with many contracts, I can now be far more careful, and choose the contract offers that will benefit me not just financially, but also what best suits my health and family.
Now, by no means do I earn any extreme amounts. Last fiscal year, I earned about 50k euro after taxes, which may not seem like much, but in terms of swedish living costs, that's well above average. However, as a freelancer, I do have to set money aside for courses, seminars etc.
(OK, I'll take that question from the clueless nerd in the peanut gallery)
Why do I set aside money for courses, seminars etc, when I could just use google and study on my own?
Well, as I pointed out above, contacts and reputation are everything if you want to be successful, and don't want to be screwed over. To the intelligent AND wise people, it also means exchange of experiences, those things you can't teach via text tutorials etc. It means getting in touch with new people who can forward things your way, as you forward things to them. In terms of reputation, one of the way it helps is that participating in courses etc lets you be seen as still keeping in touch, still able to learn, that you are not stagnating and too heavily wedged inside a niche.