Most of my users who are adding primary contacts to opportunities are instinctively or accidentally clicking "Create New Contact" because it's bold, blue and has a plus by it.

Also, the important text that shows "(Create New Contact)" is usually hidden by Chrome's AutoFill popup (which is enabled by default).

The option often shows even if an exact match has been found. e.g. typing "John Doe" would give the result of "john doe" - the correct result - and then show in bold and blue: "+ John Doe (create New Contact)" - the wrong result