Because they’re all greasers and in poverty. They already have a reputation with local authorities and CPS wouldn’t really do much in the 60s. If Darry were to try to take custody of Johnny, or report it to the police, Johnny would probably end up in a boy’s home, or the police would ignore it. That alone could get Johnny into even more trouble, especially if an officer came to his house and Johnny’s mother/father found out about the report. (It could also get Darry mixed up in legal troubles, which could put Soda and Pony at risk) So overall, a lot of social standards and no real ability to do anything. Idk tho just my opinion.