Stack Overflow has tons and tons of things which aren’t in the documentation and almost all of them are questions from people who couldn’t find the answer in the documentation – exactly what ChatGPT needs since it also doesn’t understand anything about how a given program works, only what patterns are common in public examples.
Sure, it's in there - but so is all the library documentation ever written, billions of lines of code, and millions of tutorials. StackOverflow is just one small part.