
Marketing. They call it that because they’re trying to sell it to kids who (think they) want to read “adult” books, and would be shy about reading anything classified as a “children’s book”. And of course “pre-teen book” sounds both very limiting/niche and undersells the broadness of the appeal. Also plenty of ACTUAL adults read young adult books so it’s really more accurate than much anything else you could call it since readership ranges from young people, to adults