When you look at how many new plays get written and performed every year, it is remarkable how little drama is lasting - how little of it is really any good.
I don't know how many good, lasting novels there are in English literature, from previous generations - hundreds or thousands of them.
Books that people still buy and read; books that are are worth re-reading.
But hardly any plays.
In drama there are so few lasting works that you can be a famous playwright for doing just one of them: Oliver Goldsmith for She Stoops to Conquer or Oscar Wilde for The Importance of Being Earnest.
In the whole 18th century there are indeed only three standard classic plays - She Stoops and two others by Sheridan: The Rivals and School for Scandal.
There were periods of many decades when not a single lasting play was written. I know there are several so-called 'Restoration Comedies' - but really...?
In fact, if you subtracted Shakespeare and Shaw (who were both prolific of classic plays) - drama would pretty much disappear from the literary canon.
If it wasn't for Shakespeare, who would (except from curiosity) want to sit through anything by Christopher Marlowe or Ben Jonson? Or Webster, or Beaumont and Fletcher? Not I.
If it wasn't for Shaw, who would bother with Pinero, or Galsworthy?
Plays which seemed overwhelmingly good on first viewing (for me this would include Tom Stoppard's Jumpers and Travesties, and Harold Pinter's The Caretaker) after a while seem unlikely to survive.
Indeed, in most respects, it seems that theatre is even more ephemeral than movies.
Assuming the medium itself survives, there were a lot more classic movies in the twentieth century than there were classic plays - does anyone really want to see an Arthur Miller play nowadays?
I would guess that there never have been, and presumably never will be, many really good plays.
Indeed, leave-out Shakespeare and Shaw and it would be a reasonable approximation to say that there are no really good plays at all, and theatre is merely a diversion.