Anthony Neilson nailed it in this article from 7 years ago. Are we doing better today than we were then?
So what are we doing wrong?
The most depressing response I encounter when I'm chatting someone up and I ask them if they ever go to the theatre is this: "I should go but I don't." That emphatic "should" tells you all you need to know. Imagine it in other contexts: "I should play Grand Theft Auto"; "I should watch Strictly Come Dancing." That "should" tells you that people see theatre-going not as entertainment but as self-improvement, and the critical/ academic establishment have to take some blame for that.
Many critics still believe theatre has a quasi-educational/political role; that a play posits an argument that the playwright then proves or disproves. It is in a critic's interest to propagate this idea because it makes criticism easier; one can agree or disagree with what they perceive to be the author's conclusion. It is not that a play cannot be quasi-educational, or even overtly political - just that debate should organically arise out of narrative. But this reductive notion persists and has infected playwriting root and branch.