Using a wildcard as a return type implicitly means that the return parameter should be considered as real-only but without any way to enforce this contract.
Let's take the example of method returning a "List<? extends Animal>". Is it possible on this list to add a Dog, a Cat, ... we simply don't know. The consumer of a method should not have to deal with such disruptive questions.
List<? extends Animal> getAnimals(){...}
//First Solution: the list can contain any kind of animals List<Animal> getAnimals(){...} //Second Solution: the list can contain only one type of animal List<E> getAnimals(){...} //Where E extends Animal in the signature of the nesting class