Why is it that when you watch House the doctors, especially the three junior doctors that work for House rarely have stethoscopes on them except for in those situations where they are actually preparing to use the stethoscope?

House sometimes plays with one. Almost every doctor I know seems to carry a stethoscope whenever they're in the actual office or hospital. They don't seem to go anywhere without them for the most part, with the possible exception of the MRI room where metal is not allowed.