[UnoriginalGuy]

Save space. If it had to have a "reply" button after every reply in a large comment block it would not be able to fit very many comments on the page.

They really should scrap the reply link entirely and move it next to the "flag" button e.g. | reply

[mintplant]

Mm, can't be that. If you highlight the area under the comment, there's a row of dashes ("-----") where the "reply" button should be. It takes up the same amount of space.