Shouldn't a "full stack" engineer also be fully versed in hardware engineering, microprocessor architecture, kernel and driver programming, operating systems, programming language and compiler design, etc?

You sound like someone who is not good or dislikes either backend or frontend work and cheers for anybody who says that one person can`t do work. There are many of us who can.

So on the contrary, I am a generalist and love poking around all over the stack. But that's it. I am a crazy breadth person but dont like to go into the depth at each level as that has way too much overhead for me. So that limits my expertise (but not love to tinker) in any one area. In companies/places where we need to go beyond POC apps, standardization (within stack and tech) kicks in, and you are not going to be productive if you are not doubling down on one or two areas. Worse are political problems where as an "apps engineer" where you are implementing a bunch of interfaces but diving deeply requires you to transcend org boundaries (again I am not a fan of that, just reporting as I see it), limiting your productivity.