Why don't we call doctors by their first name?

Code Blog wonders aloud:

Why don’t we call doctors by their first names? Is it a sign of respect to say “Dr. Jones” instead of “Fred?” Or is it a throwback to the era when nurses used to scramble to give up our seats when a physician walked onto the unit? These days, nurses work more closely with physicians than ever. We call ourselves the “healthcare team.” Do teammates have to address other members with a title?