There's a lot of good responses here, but the real reason is Amazon isn't set up to do it any other way. The arm of Amazon that does direct purchasing is fairly small and invite only. It's been this way since the beginning. They now have an automated program called Vendor Express that does direct purchasing, but only for certain categories and because it's 100% automated and self serve there are a lot of problems.

They are working on it though, and Vendor Express gets better every day.