Mostly because... Putting aside the question of reading levels, literacy and all the other jazz people threw in there for funsies, the teenagers I know who read? They are reading those things. They just don't read them in the published book formats you're thinking of.

The issue with all the adult material they're reading is more that there's nobody talking about it with them. Nobody is working through that with them, because adult books/materials and teenage readers. Also one can love MG books and also read and like more complex material. And also adult books.