Because sometimes we're busy? Or we're so tired or uninspired, we don't have much else to say other than "COOL!" and figure such a bland comment is worse than no comment at all, esp for a popular artist who has to deal with a lot of comments? Or because sometimes art moves us in a way that is difficult to express in words? Or because they just don't feel like it? Also, not everyone on DA speaks the same language as the artists they like. There's nothing rude about it, it's just different strokes for different folks.