Physics specifically has a long tradition of expecting a wide breadth of knowledge. For example, in every physics department i know details about, the expectati

Physics specifically has a long tradition of expecting a wide breadth of knowledge. For example, in every physics department i know details about, the expectation is that any professor is able to teach any class, and class assignments indeed change very regularly. I think part of the reason is that the knowledge tends to be "use it or lose it" on a multi-year timescale. If you work with physicists who have been pidgeonholed in industry for decades then this is probably to be expected.