Because every Jim, Bob, and Larry can post on the internet. You ask a question on a forum, and you could get two or three really knowledgeable people give you an informed answer, but you could also get a dozen other schmucks just making stuff up.
Books are usually going to give you more detail and context as well, and a more complete reference. Sure, I could quote you a sentence or two from Mine to Mint in answer to a question, but you'll get a lot more complete answer if you read the whole chapter which explains that snippet.