Bike Buying Advice

I still only have to tell about five people a year that the bike they have is the wrong size. It's a difficult conversation, but in their heart of hearts, the customer already knows it. It seems I'm merely there as an independent party to justify incoming purchases to their significant other. I'm happy to oblige. So it raises the question: why bother writing about the difficulties of bike sizing, if most people are finding their way to the correct size anyway?