When I first started out, I wasn't sure how everything worked. Ricardo_Klement was the first to comment on one of my memes, and I wanted to reply with a meme comment (which are only available after 1,000 points) - so I waited until then to reply.
Another one could be that they comment on their own meme thinking you will be notified, and never even use the "reply" button.
I don't know, there are a plethora of possible reasons. But newer users do need comments on their memes, to help bring them in to the community :)