These gradual type checking frameworks allow migrating massive code bases incrementally. Doing a complete big-bang rewrite into a different language isn't feasible (for the obvious software engineering and business reasons).
Another thing to keep in mind is that type annotations can be done incrementally. You can't take 1000 Python files and port them to Go one by one, but you can add types a little bit at a time with a completely usable codebase at every intermediate step.