Well, to be honest, not a lot of people have the time to do that. Most people read and appreciate the comments they get, but can't always reply, which is fine, you know? People have more important things than deviantart usually, especially if they get many many comments. Other people just don't want to reply to things like "Cool." or "I like it." Those comments are usually a little annoying and there doesn't always seem to be a reason to reply.