I'm not sure, but I noticed if I remove one (by answering it) then a new one shows up at the bottom of the page. Perhaps the original programmers thought one page of zero replies is the right amount? I know when I go through the list, some of them I don't reply. With this design those will sink down the list and disappear.
I had assumed this might be an expensive database query. But I have definitely run out of zero-reply threads I wanted to reply to before I was feeling done in the past; it would work for me if this was a typical paged list that let us go further back in time.