> That out of the way, I've never met a perfectly competent "full stack engineer.". Most seem to learn just enough to hack both together.

It's mostly due to the sheer amount of things you have to keep in mind at any given time. Not the core things like algorithms and such, but (Javascript + Typescript + React + Redux + GraphQL + Apollo + .... ) + (Java + Spring Boot + MySQL + BigQuery + PubSub + Datastore + ...). All the different libs, and the ways to communicate, and idiosyncrasies specific to different languages and different libs.

There's never time to get good at something, as you're pulled apart :)