Keep in mind there is also the chance of false positives on those types of tests. So anything you can do to do them only on people where there is a higher chance of them being meaningful saves not just the cost of the tests but mental anguish for people who would get the false positive results.

I agree, it's not an exact science and I am not a doctor or run a practice so I do not know why they would do it this way. Probably a "best we can do" type situtation.