Not every person that reads a thread has an answer to the question, comment about the picture, or something to share about what the OP said. A title does not always tell the story and after reading a thread one may either just not feel like typing, feel that the post is just too long and time consuming to get into, see that others have already given the person similar info, or just may not feel that it is worth it to go there.
Also, there are many people that cannot post because they are not paid members. But they do have access to several of the threads. There are many reasons why a thread has more views than comments. If I commented in every thread I see here, I would be spending way more time here than I should.