Except when they do. For example, when 100% of men above a certain age are screened for prostate cancer, or 100% of women above a certain age are screened for breast cancer. Both cases spawned major public health campaigns to encourage screening, followed years later by recommendations AGAINST 100% screening, based on the high degree of false positives and unnecessary treatment.
Other cases that come to mind:
-- doctors who offer "full body scans" as a part of an executive physical; you're pretty much guaranteed to turn up something that is 2 sigma away from the population norm, somewhere in the body, on such a scan
-- spinal x-rays for back pain. Doctors almost always find something abnormal, and use that to justify the back pain and treat aggressively. But, we don't really have a good prior; if you x-rayed 1000 people off the street, would we find similar abnormalities frequently?
Other cases that come to mind: -- doctors who offer "full body scans" as a part of an executive physical; you're pretty much guaranteed to turn up something that is 2 sigma away from the population norm, somewhere in the body, on such a scan
-- spinal x-rays for back pain. Doctors almost always find something abnormal, and use that to justify the back pain and treat aggressively. But, we don't really have a good prior; if you x-rayed 1000 people off the street, would we find similar abnormalities frequently?