I like the idea of using Java sometimes. But why does it have numbers and arithmetic? Java should be great at tasks like, "cats are a kind of animal", or "my drawing program can draw both squares and circles." Arithmetic seems so unnatural in an object-oriented language. ;-)

Perhaps Prolog isn't ideally suited to a lot of programming problems -- that's a personal decision to make -- but it's silly to wonder why a general-purpose programming language would have general-purpose data structures.