If all of those brilliant plays were written by the same person then we want to acknowledge that person for his or her great accomplishment. Why? Because it's another testament to humanity's great potential.
A great accomplishment by itself is fine. Take the E=MC[squared] equation. If it had been arrived at piecemeal by a larger number of scientific minds it would be just as useful but we wouldn't have the cult of personality which has grown up around Einstein. Knowing who that person was teaches us more about who we are and what we can be.
And people who achieve great things are eventually so studied or admired that they ascend into the realm of myth or legend. Anyone who values mythology should be able to see why it's only natural that we remember, and honor, the people who made great contributions in the past.