Probably the same reason their ads don't state "people with high blood pressure need not apply", "people with a low hematocrit need not apply", "people who have taken aspirin in the last 72 hours need not apply", "people who inject insulin for diabetes need not apply", "people who currently have a cold need not apply", "people who have donated blood recently need not apply", "people who have recently been vaccinated for smallpox need not apply", .... How large are those ads, anyway?
When I donate, there are about forty to fifty questions like this, and I've actually been turned away for three of them. I keep track of this, because one of the questions involves prior deferrals, which they then check for the possibility of coming up again.
They automatically ASK about those. I don't know what their automatic deferral policies are (except that a "yes" answer to "have you ever been deferred" does not seem to mean an automatic deferral again). And I don't know how many of the people working at the blood bank actually know the reasons behind all the questions on the list.