The answer is simple:
extends is how you spell "is a subtype of" in Java. The declarative semantics of
implements is likewise "is a subtype of", but operationally it further means "provides an implementation for", and is a relationship specifically between a class and an interface. Types as such don't implement other types.
By the same token, interfaces
extend their superinterfaces and always have. They also
extend Object, which is a class.