How the characters in "Waiting for Godot" made so believable to an audience, considering their exaggerated stupidity and lack of intellect?
I take it you have only read the play and not seen an actual performance.
A version with Ben Kingsley at the Old Vic in London sold very well.
Existentialist plays like "Waiting for Godot" and "No Exit" can be tough sells unless the actors are very good. "Waiting for Godot" is a study of man's existence and the search for hope in it.
The characters are not real people they are vehicles that converse about philosophic issues in a gritty and tenacious manner. Are the generality of human beings on this earth particularly intelligent or intellectual? In this the actor must perform them in a very human way.
Human's existence is a very difficult philosophical point. The play's characters are held suspended in a life that characters struggle to find any purpose or meaning to grab onto.
Who are you? Where does the world come from? Is there reason to hope?