What do you mean when you say tester? If you mean "someone to run manual test cases," it is probably because those sorts of jobs are pure tedium... They can be outsourced to websites too.
If you mean a software engineer that designs software to validate that other complex software systems function (often dealing w nontrivial engineering efforts and undecidable problems) , they probably don't freelance because we denigrate them by calling them "testers" and they'll get hired fulltime by companies that understand the value of their work.
If you mean a software engineer that designs software to validate that other complex software systems function (often dealing w nontrivial engineering efforts and undecidable problems) , they probably don't freelance because we denigrate them by calling them "testers" and they'll get hired fulltime by companies that understand the value of their work.