forum/chat are different in the way they are referenced across long periods of time. A forum thread will show in search results for someone asking similar questions, or searching similar topics, but in chat this is not the case. Additionally, a forum has capacity to stabilize very large and detailed conversations about a single topic, where chat is usually done in the form of small 1-line sentences, and a much more casual dynamic even for serious topics. These things serve two entirely different purposes, so both are available. It would be strange to find someone attempting to use a forum as a chat room, and vice versa.