No you're not stupid. This question actually comes up a lot on r/books on reddit. The best answer I've seen is reading is alot like eating. You probably can't remember what you ate last week but it kept you alive and healthy. Personally I believe you can't really tell how a book changes you but I believe every book a person reads affects them a little bit. When you finish a book you don't need to write a 5 page essay on it. If you enjoyed it then it's worth it. Or you could take notes on or annotate stuff you read. That's also an option if you really want to be able to call back on stuff.