It is for the same rationale that the uncomfortably tiny font size of HN makes sense; only from this zoomed-out perspective can you feel the shape and texture of the discussion, instead of being overflowed and taken away by the walls of texts.
I’ve been struggling to find an efficient way to read Hacker News and now it seems to me the best way is to skim thru threads and be aware of how the topic develop around (and from) the original linked-to article, but not to get too much into comments, since the most valuable thing you can get from HN is what are being thought by hackers currently rather than what their thoughts are.