Why an interface can not implement another interface?

implements means a behaviour will be defined for abstract methods (except for abstract classes obviously), you define the implementation.

extends means that a behaviour is inherited.

With interfaces it is possible to say that one interface should have that the same behaviour as another, there is not even an actual implementation. That's why it makes more sense for an interface to extends another interface instead of implementing it.

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On a side note, remember that even if an abstract class can define abstract methods (the sane way an interface does), it is still a class and still has to be inherited (extended) and not implemented.

implements means implementation, when interface is meant to declare just to provide interface not for implementation.

A 100% abstract class is functionally equivalent to an interface but it can also have implementation if you wish (in this case it won't remain 100% abstract), so from the JVM's perspective they are different things.

Also the member variable in a 100% abstract class can have any access qualifier, where in an interface they are implicitly public static final.

Conceptually there are the two "domains" classes and interfaces. Inside these domains you are always extending, only a class implements an interface, which is kind of "crossing the border". So basically "extends" for interfaces mirrors the behavior for classes. At least I think this is the logic behind. It seems than not everybody agrees with this kind of logic (I find it a little bit contrived myself), and in fact there is no technical reason to have two different keywords at all.