The thing is, even in her "adult" novels, Rowell's protagonists are struggling with relatable issues. Sure Georgie is dealing with her marriage and her job pulling her in two different directions, but that is not so different from what Eleanor, Park, Levi, or Cath are dealing with in her YA books. Like in the above fanart, Georgie is stressing about a phone call from her husband. She could just as easily be a 17 year old, waiting to hear back from the boy she likes. The situations are relatable, even when the character's age differs slightly from the reader's.
Also, Cather and Levi from Fangirl make a cameo in Landline and it was too cute for me to handle.
Rainbow Rowell has won two literary awards: Goodreads Choice Awards Best Fiction (Landline) and Goodreads Choice Awards Best Young Adult Fiction (Eleanor & Park). She was also nominated for Goodreads Choice Awards Best Young Adult Fantasy & Science Fiction (Carry On).