Credit where credit is due

I don't think West Side Story was credited to Shakespeare either, but I'll assume what happens at the end of this movie... happily ever after!

Oftentimes, both authors and moviemakers do not give credit to the sources that have inspired their works. I don't know the exact rules for films and literature, but for the visual arts, the artist must create a piece of work that differs from others' works by at least 10%. So, up to 90% of a piece may be the same as the original source of inspiration.
By the way, did you know that Shakespeare's "Romeo and Juliet" is basically Ovid's "Metamorphasis," but written centuries later? I've also heard that "Star Wars" (the original trilogy) is based on the hero story, which may be traced back to "The Epic of Gilgamesh."

True, but then Romeo & Juliet is based on an old Italian folktale, as are all of Shakespeare's "Italian" plays, with the exception of Tempest. The Tempest was his only "original" work. Ol' Bill's talent lay not in coming up with the story itself, but telling it better than anyone else.

Plus, the theme of two young lovers from warring families/factions/cities/nations is an old theme, that's been told and retold in many different ways. I think it's nearly always a story worth telling, and is universal in its message, so we're always eager to tell it, to hear it, to watch it, and to talk about it.