Maybe they are. My understanding (and this may well be false) is that show HN is more for cool tech demonstrations and side projects, whereas rate my startup is for launching and rating startups.

Rate my startup may well be hidden in a subset of show HN though.

Another reason may be that they don't get voted onto the frontpage? A quick search shows that there are quite a few show HN posts, but they don't get traction. https://www.hnsearch.com/search#request/submissions&q=%22sho...