To be perfectly honest, a lot of your complaints sound like, "Why do I need someone who is an expert in plumbing to fix my pipes?" "Why do I need someone who is an expert in medicine to diagnose my illness?" "Why do I need someone who is an expert in development to develop my software?" And the answer almost always comes down to, because they've been doing this a lot longer than you have; they've been studying it more, and they know more about the pitfalls and potential issues than you do.