Why can’t we use main account for forums too?
Because of user passwords are hashed with salt, we can’t automatically create a forum account from existing accounts. We can only do this, create a forum account with a random strong password and send an email for password reset. But users might think someone tries to steal their accounts when they saw password reset email without their action.
So we thought that the best thing to do is to send invites who have already registered on our main site and also to new users. In this way, users can select their passwords for the forum. Actually, from the user’s perspective, it will be the same effort to reset the password and accept the invite.
In the end, you are right. Forum and site accounts are separate and will stay that way.