To add to the point on testing - with the mainsteam labs the people examining the stools often can be the lower end of the paid spectrum of employees, meaning they're not even expert parisitologists.
If they find something that isn't obviously a Tapeworm or a pinworm they may even dismiss the sample judging by the results people get with these labs.
When people find flukes and can identify it in their stool by refrencing with parisitology literature that is good evidence right there, and if a corporate labs barely have people spending much tome examining samples sent to them it isn't much help.
People have even sent samples with worms put in them to some of the corporate labs and got a report back of negative.
To add to that even more the blood tests in the U.S. at least, cover a small spectrum of parasites, so if someone has a parasite that may not be the most 'common' ones the labs and even CDC may not offer them which is telling in itself.