That's because it is limited. It's not meant to be a way to provide polymorphism. One of the things that it does provide is the knowledge of every possible case

That's because it is limited. It's not meant to be a way to provide polymorphism. One of the things that it does provide is the knowledge of every possible case and a reminder from the compiler if you missed one. This is not possible in many OO language. FP languages have different ways to inject new behavior. You could for example define a function a -> (a -> Int) -> Int. This function now works for any type for which you can also provide the "interface" a -> Int.