Because when you do every website forces you to register with them. This process is absurd, cumbersome and hurts sales.
When was the last time you went to the store and they said “well, before you buy any of our products you must fill out this registration form”? Unless you’re taking out a loan or obtaining a warranty with your product, then it’s unnecessary.
This is especially important in the internet world where sales are based upon short-lived sales. The times I have been to a site and left because I was forced into registration in order to use or buy the product is countless.
What if “Buy Now” really meant ‘Go to the check out counter, swipe your card, and you’re done…’