Cloning requires copying. Sharing requires locking and triggers on any updates so the updates get applied to the other thread(s). Clearly, nobody has yet written the code to put all of the locking and triggers in place for globs.

Now, there certainly could be more to it than that. But I can certainly understand writing the code to lock and trigger for scalars before writing the code to lock and trigger for globs (since that code for globs isn't of much use if you don't have the code for all of the items contained in a glob already anyway).