Well, in the hard science majors, there's considerable training, which is necessary for a large fraction of occupations. Granted, a physics PhD who became an economist may have been signalling, but it is far from the norm. What is the norm is that vast majority of individuals employed as physics PhDs would be unable to perform some parts of their work if they hadn't undergone relevant training, just as you wouldn't have been able to speak a foreign language or drive a car without training.