The problem is that Python type hints are not enforced at runtime. They can can not be trusted. You have to rely on external type checkers that are relatively slow and different checker might give you different results.
Probably the only mainstream backend-language getting gradual typing right is PHP. Yes, good old PHP. Dead simple type system, no productivity-sucking compile step like in Typescript, actual runtime enforcement of types contrary to Python and great static linting.
Though PHP is also a fair bit less expressive and dynamic compared to both Python and Ruby so it can get away with an much simpler type system. Then again, as someone who has to work with many legacy systems, sometimes I am very grateful for the lower expressiveness.