How would I demonstrate that the product is falsely labeled? I still don't have a lab.
How would I pay for the experts to testify against Wal-Mart? They might hire a lawyer to disagree.
How would I establish standing? My actual damages are limited to the amount I've personally spent on the fraudulent products. Oh, maybe I get a class action certified! Now I need dozens of lawyers.
Who does Wal-Mart pay the damages to? If the answer is "the class members" you really mean "mostly the lawyers".
Without an agency funded and tasked to do this, it's not worth anyone's time and effort to do it themselves.