PhD parents still have trouble with elementary basics simply because they do not train or keep up the basics. It's no different than professional athletes who play at a high level but still goof on some of the basics. PhD just means that a person is specialized in a particular area. Basics builds the foundation and like everything else in the world, basics change.I don't think it's really an issue of someone at PhD level going back to elementary school. It's possible to go back to a form a few years later and look for something new to learn from it. You probably can't do that with the reading primer from early grade school (unless, of course, you're trying to learn how to teach kids to read, but that's another matter, entirely).
Here's a perfect real world example. Typing skills was not considered basics 40 years ago. Now it's a Basic. People who are older often have difficulty typing where as for younger people it's almost natural.