Because when you are having 1 thread of discussion amongst a dogpile of others it helps to be able to reduce the post to only 1 person until you can find the right point in the discussion, then you can bring the rest of the thread in to get the full context, it would be even better if you could simply screen some people out to thin the thread down.
If everyone used the reply button consistently it might not be such a problem, but since the same person often replies multiple times to one post or consolidates responses to multiple people in either a general reply or a reply to one specific person trying to follow a conversational thread through the links is not always easy.
Plus I like only reading what I have written without all of the plebs cluttering the place up, nothing like a whole thread of me pontificating to remind me how great I am. Excuse me while I find a device to deflate my head.
TTFN,
WK