The downside of scanning for everything is that you'll end up increasing the cost of medicine considerably for extremely little gain. Case in point: breast cancer screenings catch a lot of malignant tumors in an early stage helping many patients before their cases become more serious. But the downside is that it has caused a large number of un-necessary procedures as well raising the costs and - and this is the kicker - blocking the available capacity with cases that would never become serious in turn denying that capacity to people whose cases are serious.

A bit of googling will give you lots of articles on the subject, the same kind of evidence exists for prostate and other cancer screenings as well as other diseases.