The reason is that the community decided they needed some form of type annotations, but nobody could agree on which ones or how they should work. So some syntax was added just so the parser wouldn't fall over, but the semantics were left to other projects to "let 100 flowers bloom".

This worked about as predictably as you would expect. If you grabbed a random package from pypi, suddenly your choice in typechecker might complain about its API if it wasn't made using the same typechecker as you, so a separate, "standard" choice in typechecking was made called mypy. But they couldn't break older code not using mypy so it's strictly optional.