I doubt you'll find anyone here seriously saying that we've found a definitive theory of metaethics. That is our eventual goal, yes, but right now, there are at best several competing theories. No absolutely correct theory has even been proposed, much less endorsed by the majority of LW. So the answer to your question ("Why do people believe it?") is, as far as I can tell, "They don't." My question, however, is why you think this is something really bad, as opposed to something just slightly bad.