Again, there are no shortage of adult fiction books without "dark themes". There is quite literally tens of thousands of books outside of the extremely narrow YA genre. If you think you need YA books for a carefree and enjoyable story, you're just wrong. Try Hitchhikers Guide to the Galaxy, for example. One of the most popular goofy and lighthearted stories in the past 50 years. Yet still clearly written for an audience outside of 12-18 year olds lmao. The dark themes and problematic topics you're getting all hung up on are not a requirement. It's purely about who the author is writing the book for. YA books primarily feature teenage protagonists in order to relate to the experiences of their intended audience. Why adults are so obsessed with reading books meant to appeal to teenagers and the growing pains they are facing is very strange to me.

There's no shortage of easily digestible adult fiction out there. I'm not saying you have to only read Holocaust survivor biographies as soon as you reach adulthood. For example, I read a lot of WH40k fiction. My reddit username is a character from the Eisenhorn novels by Dan Abnett. They're not challenging to read in the slightest, yet they're still very clearly written with an adult audience in mind. I just don't really get why a well adjusted adult who can comprehend higher level reading would deliberately choose to limit themselves to YA.

Nothing wrong with reading a book from your childhood or revisiting a series you never finished. But I do find it weird when adults read exclusively YA. Unpopular opinion around these parts for sure. But YA is its own genre for a reason. Everything is much more black and white, problematic topics are avoided entirely or handled with kid-gloves, etc. I just don't see the appeal of reading literature written for children as an adult.