I treat FWR threads as conversations. And because they often move quickly, I rarely look past the front page.
I think most of us don't bother looking past the front page. So we might miss a topic that's pushed off the page. So that's one thing.
The other thing is, I understand the thread doesn't go away and can still be read, but if you see it as a conversation, it would feel a bit off IRL if someone buttonholed you and said "I heard a recording of a conversation you had 2 years ago, I'd like to ask you some questions about it."