Because most chat conversation is clique-ish and the room tends to be ran by the clique. Also some people aren't able to sit and chat in a real time conversation. And its somewhat limiting to have one person at a time to talk to. Or confusing to have a few. This allows you to get thoughts and jokes out on multiple topics on your time, without feeling pressured to hurry and respond.
I think forums allow for a more in depth chat and, in some ways, allows you to get to know others better than in chats. You can see a persons view and attitude in many different areas throughout the various types of forums. Its actually less shallow than much of what goes on in the chats. Though one on one chats are not less shallow by nature. But its so difficult to find someone to chat with that isn't looking for a mate or a donation and that won't freak out and call you a creeper if you say hi. Look at all the PM messages saying 'don't PM me'. Some are friendly, some are quite rude and nasty. Who wants to sift through all that in hopes of finding one good person?
Actually to be honest, I find forum posts to be more of a mask than chat. Forum posts are public, and I would suppose people would tend to be more wary and guarded.
Also, I've found that just the written media, can be misconstrued and sometimes other people may not get the depth of what is expressed. There are lots more chances for people to be contentious, even if the intentions of the poster was not likewise.
The chat allows for real-time audio and even visual input and somehow, that makes the person even more ''real''. I understand that some of the rooms go a bit fast and stuff but I've found most people quite friendly. To be truthful, I've learnt more about the Bible and God talking to people on chat.
Sometimes someone will just pull up a Bible verse and others would read. To me, it's easier to pray and encourage people on chat, when at that point of time, they're unguarded and being honest about all their feelings and emotions.
I've made many friends on chat. There's just no 5 minute time to edit what you say