Bluntly, I don't want to read every comment.
For instance, the top comment at the top of the page may have 10 karma and 50 replies, while the next valuable comment has 15 karma (but was submitted 3 hours earlier), and isn't visible until halfway down the page. Without the karma displayed, it's much more difficult to skim the thread. And for all I know, the rest of the comments only have 1 point.
Plus, I recall from my reddit days that the earliest comments are way more likely to receive votes. So I always ended up trying to divide the karma by the time since post, which just distracted me.
Different strokes I s'pose.