My guess is that they close the thread once a resolution has been made (online or offline) so others won't then post their issue on the same thread instead of starting a new one.
Think about it this way:
You answer complaint emails at your work. Instead of each customer submitting their own email to you for follow up, they find an old email that was resolved 4 months ago and put their issue at the end of it. Now you have to go through all these old emails to find out if you need to handle something or not.
Or:
You get an email, you resolve the issue, you file it away. You get a new email, you resolve the issue, you file it away.
I know which I'd prefer.