HN is very minimalist in its UI design, almost to the point of cutting off its nose to spite its face. It lacks a lot of navigational elements and affordances other forums and sites take for granted. They don't even paginate long threads anymore, they literally rewrote the entire backend language to be able to render threads efficiently enough to allow them to remove that feature because for whatever reason having thousands of comments on a single page is better.
It's probably assumed that scrolling up will suffice without adding another UI element, or maybe there's a browser shortcut you can use.