Why do doctors in the us dismiss and fight the possibility of parasitic infections often diagnosing a patient with delusional parasitosis without running a single test (often with zero psych background i should add). is this cultural? fear? ego?

It is a little more complicated. First, parasitic infections are rare in USA and a positive test in the presence of typical symptoms is needed. Second anti parasite medicine were used indiscriminately for prophylaxis and treatment of COVID due to Internet/media driven beliefs and that created suspicion in prescribing liberally such meds that may have serious side effects.

Formication, the sensation of insects crawling on the skin, once rare, has increased dramatically with the increased use of methamphetamine. This is a medical problem, not psychology. It manifests as "skin picking," causing numerous sores all over the body but sparing the area between the shoulder blades. It's worse during withdrawal. True parasites are easy to detect and should be considered.