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.