Slate has a long essay deploring the practice. The New York Times declares the end of adulthood in America. Oohh. that sounds really bad. Even I surely remember riding the MBTA home from work with every one in the car reading the latest Harry Potter.
Why do adults read YA novels? Simple, the last decent main stream writer was Ernest Hemingway, and he died 50 years ago. The main stream writers of today are so boring hardly anyone can stand to read them. Main stream novels don't have heroes, they have wimpy anti heroes with neuroses, the other characters are unpleasant, nasty, and ineffectual. They are set in unpleasant locations and the characters do little other than feel bad about things or make others feel bad. Who wants to read that?
The YA books, Harry Potter, Tolkien, the Rick Riordan Olympian stores, the Hunger Games books and others have heroes that overcome difficulties and save the world. At least Harry Potter defeats Voldemort, Frodo Baggins destroys the Ring and the Dark Lord along with it, Percy Jackson destroys Kronos, and Kartniss Everdeen saves her village, if not quite the whole world yet. The characters are likeable, the kind of people you would like to have a friends. The settings are glamorous, the scenery is dramatic. They are fun to read, and come to a satisfactory conclusion.