Why can't static methods be abstract in Java?

Because "abstract" means: "Implements no functionality", and "static" means: "There is functionality even if you don't have an object instance". And that's a logical contradiction.

Poor language design. It would be much more effective to call directly a static abstract method than creating an instance just for using that abstract method. Especially true when using an abstract class as a workaround for enum inability to extend, which is another poor design example. Hope they solve those limitations in a next release.

You can't override a static method, so making it abstract would be meaningless. Moreover, a static method in an abstract class would belong to that class, and not the overriding class, so couldn't be used anyway.