Actually, I think you missed a major explanation. Shakespeare - by virtue of being a major cultural institution - gets his plays preformed at a much, much stronger level of competence than any given random playwright. Even enthusiastic amateurs, when putting on the scottish play are not merely working from the words on the page, but from a living tradition of who the characters are, and how they should be played for maximal impact. There is no such thing as an actress who has no understanding of how to play lady M.

And this fact, that everyone preforming his works are steeped in them, makes the plays seem stronger than they are - After all, the performances are typically stronger, so that must be to the credit of the works, no? Well, no. It's to the credit of the tradition and the work. Which cannot easily be separated, because the tradition didn't glomp onto Shakespeare at random.