Declaring types only where one wants to introduce a constraint would be nice, but unfortunately calls with obvious wrong types like "foo(5, {})" are a minority; in most cases types in a call depend on types of the calling function's parameters, and the type checker can only deduce unknown type from unconstrained type.

Moreover, the extremely dynamic execution and lack of compilation in Python means that determining in a lint step whether "there is no + operator for int and dictionary" is essentially impossible: if you find some __add__ or __radd__, you need to trust its declaration that it accepts certain types or not and hope it's still there at runtime; if you don't find it, it might exist at runtime when operator + is actually used.

A compiled and sufficiently static language, on the other hand, can inspect source code and libraries and reason about potentially infinite sets of definitions and prove that functions exist or don't exist.