Java does not allow you to create a class that extends an enum type. Therefore, enums themselves are always final, so using the final keyword is superfluous.
Of course, in a sense, enums are not final because you can define an anonymous subclass for each field inside of the enum descriptor. But it wouldn’t make much sense to use the final keyword to prevent those types of descriptions, because people would have to create these subclasses within the same .java file, and anybody with rights to do that could just as easily remove the final keyword. There’s no risk of someone extending your enum in some other package.