Would not they check the passenger list for Dr's first? I remember being told that this was an occupational hazard for Phd's that you might be assumed to be a MD Dr.
As a physician and government traveler, the online booking service never asks for my title, and even if it did, I'd have to choose: Dr/MD? LCDR/USN/MC? Most identities online don't require a title and I don't offer. It only induces spam.
Just as an entertaining aside - the British Airways registration page currently includes Viscountess amongst other titles. Even so it's relatively limited - in its early days it contained a bizarre list of titles including the, um, uniquely prestigious "pope". Presumably the result of over-enthusiastic data population of the DB from some authoritative resource.
Just taking a random guess, but it could be that it's easier to use the intercom and ask everyone instead of digging out the passenger list and scanning to find someone whose prefix was 'Dr' (if such info is even included on a passenger list). This is just speculation, though.
I think that's the reason why. Also if there's no physician on board but there's an EMT/nurse, you wouldn't know that from the passenger list.
Plus it's entirely possible that even if there's a doctor, she isn't listed as one.
ObTrivia: In the UK it is convention that surgeons drop the Dr title and revert to Mr/Mrs/Miss/Ms.