The reason why the first books seem simplistic and the last books so dense is because JKR intentionally wrote it for the development of her audience. In the first book, a young child really wouldn't understand what's so bad about living under the stairs, it probably seems like oodles of fun to them. Looking back, as you did, they would realize the importance of what Harry endured. But as the audience gets older, JKR's writing becomes more advanced, as it should. I don't know about you, but I definitely would have stopped reading if the difficulty remained at an elementary level. To me, its not simplistic but brilliant, as Harry would say.