There are an awful lot of PhDs out there who are at least as incompetent as some of the physicians you've come across. Don't use such a broad brush. I know, I know, "It's fun." :-)

E.g. A student taking their final oral exam before being awarded a PhD in physics:

Q: Is the wavelength of light bigger than a breadbox?
A: ?

True story. :-(

It was a theorist, of course.

Cheers,
Scott.