Shakespeare invented most of the modern English language. Keep that in mind. And if you've ever read a Shakespeare play (assuming you're able to understand some of the older terminology, like the word "nunnery"), it's not too difficult envisioning why Spielberg isn't nearly as awesome as Shakespeare.
There's a difference between making something with "mass-market appeal" and making a masterpiece. Both have can be very popular, but there's a monumental difference between E.T. and Hamlet.
Also, to my knowledge, Spielberg did not write any of the works that he directed or produced. That's a key difference that places Shakespeare in a higher category of talent.
And finally, Spielberg is no better than Lucas in the sense that he revised earlier films and fundamentally altered their artistic content. I don't believe Shakespeare ever did that; rewriting a play is not quite the same, as that is done before the release, which is exactly what Spielberg should have done if he wanted all of the guys in E.T. to hold walkie talkies instead of guns. Art is supposed to portray a message, and when E.T. was made it really wasn't a big deal to have guns in a children's movie. Frankly, that's important from a historical standpoint. It demonstrates our continued spiral toward absolute insulation of our children, but that's a debate that can wait for another thread.