Because they aren't looking for them. 99% of all their big money making diseases are based on parasites. Why would they want you or your brainwashed doctor/nurse to know???
Just do the most effective anti parasite protocol around (Dr. Clark's WITH zapper necessary) and be rid of your parasites any and everywhere, and stop wasting money on stupid tests that reveal little to none of the real infestation level. Oh, big pharma hates that---all those profits down the drain.