system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why don't books have ratings like movies, video games, and music (explicit tag)?

Was a ratings system ever seriously considered for books?
gpt
There's actually no good reason aside from nobody has ever lobbied for it, or has bothered to create an entity like the ESRB/MPAA/RIAA to apply and enforce the labels. 

I assume it also has to do with the stereotype that book consumers are more "mature" than people who consume other types of media.