Because he asks questions but doesn’t answer them. When someone offers an answer to a question, you can disagree with them, and even dismiss them if you don’t like their response. When someone merely poses a question, you have to respond and think for yourself.
Shakespeare asks those huge, open-ended questions about massive themes: not only does he confront love and death, quintessential human experiences, but he also deals with power dynamics. What do you do when the wrong person is the person in charge? Are some people in some ways lesser than other people? How does a Jew, a black person, a woman or a bastard navigate the world? All of those questions about racism, power and misogyny are unbelievably relevant to us today. Shakespeare is asking all of those questions hundreds of years ago, and problematizing our ability to respond at every turn. It’s precisely because he is so complex that he is so endlessly interpretable for new contexts.
“Those questions about racism, power and misogyny are unbelievably relevant to us today. Shakespeare is asking them hundreds of years ago.”
Moreover, the cult around him means that each new generation has a new set of Shakespeare productions or re-interpretations. Every serious actor or actress wants to prove themselves in a Shakespeare play, and directors want to contribute to a great tradition. He is constantly reinvented for new historical eras, and for new cultures and places. He has been re-interpreted, across the world, in film, dance, opera, music, and image. There’s something profoundly versatile about him; each new culture can remake and remold him to their ends.