Because a lot of things implement the interface Iterable or extend it as a sub interface.
The implementing classes are:
This is a huge list. And it touches on all sorts of packages out there.
Furthermore, you want to minimize circular package dependencies. If a class in package A depends on a class in package B which depends on a class in package A, you've got a circular dependency. They're not always bad that they exist - but they lead to other circular dependencies and that can be a bad thing. Its not bad by itself, but it is a design smell that indicates that the coupling between two classes or packages is too tight. It is the start of technical debt accumulating.
The solution to this is to say "yes, the Iterable interface is something that is depended on in a wide variety of classes and packages throughout the entirety of the java and javax structure. It should be in the most base of the language libraries - java.lang."
And that is where you will find it.