It also takes a lot of expertise to understand why certain subjects are introduced in intro courses, and to frame those ideas in a way that will help those stud

It also takes a lot of expertise to understand why certain subjects are introduced in intro courses, and to frame those ideas in a way that will help those students learn more advanced topics. It doesn't require Ph.D. level knowledge, but it helps a lot, especially when you're talking to one of those few students with more and deeper questions about the subject.