The illusion of exclusivity & the intrinsic value (translation: $$$) it adds. Of course they don't want their oil sold at a discount retailer...their margins would go down when they were forced to discount it.
There once was a motorcycle shop just down the road that Amsoil MC oils at a very good price, and I figured he'd probably be able to order in some car oil for me, but then they went out of business. Just not meant to be!
Fortunately I do have a local source for Redline, although it's pretty pricy too. Their 5W-30 has a NOACK of 6, 5W-20 is 8.