Freelance web designers and SEOs are everywhere because you don't need any experience and often times don't even require any actual skill to do the job. All you need to know is how to repeat the whole "Social media marketing your business reach customers user experience blah blah nonsense" spiel and people say "Yeah! I want to miraculously reach the top of Google for everything and make a billion online like Zuck" and hire some dude for $300 who read "HTML and CSS for Dummies" last week at a Barnes and Noble. I know this because I used to be that asshole (not the business owner, the freelancer).
Other kinds of web/tech freelancing have very simple-to-understand values. In the minds of a lot of people who hire freelancers, Web design = Branding or Marketing = Profits. Simple. Not exactly correct, but that's the line being fed to everyone these days. Same with SEO. SEO = Rankings = $1B profit.
Testing is different though. I have to say however that if a company even needs to test software to begin with then they're probably playing at a higher level than the companies hiring freelance designer/devs and SEOs. Still, the value of a tester is still nebulous. You don't hear freelance testers out there drilling "Testing will make your customers love to use your product, increase efficiency on yours and the customer's end, and save you a ton of money" or something equally easy to market. To most businesses testing means "we played with the app on the staging server and it worked fine for us. time to push it live". It just comes off as an extra expense that isn't worth the investment.
And why is that? Because freelance testers, unlike other freelancers, have failed to create a consistent, marketable appeal to business owners (i.e. tell them why it's going to make them tons of cash) the way other freelancers have. And it's probably because those who do testing are a little more thoughtful and less about pizazz and flash than designer/developers and the nature of testing itself is kind of boring unless you're really into it yourself. Design is fun because everyone wants to be an artist. SEO is fun because people are given the idea they're in control of which keywords they'll pop up for and such. Testing? Totally boring. Some guy's gonna come in here and find bugs in our software? We can do that ourselves or just make sure to mark all the complaint emails as TODOs in Gmail.