It would pretty much remove the forum’s purpose. There is an unofficial Roblox forum somewhere (the fact that it’s unofficial may kind of influence this) and everything is open to the public. People are spamming things like “what did you guys have for lunch today” or “what should I do when im bored”.
If #lounge was opened to members, it would also become this sort of “#spam” category. The mindset behind the lounge, back when PA still existed, was that you have to earn it somehow. You had to be active in the actual development section in order to chat about other topics.
Since PA is no more, you can’t become a regular anymore and therefore can’t earn it. You have to do virtually nothing to become a member, and allowing this would basically make the #lounge category public.