-> Because some users are returning users already informed what the product is. For them is convenient to see button right away.
-> Because if you place that button decent and it does not damage your design, its not bad to have it after multipole sections so users have it always available.
"Buy now" or any other Call-To-Action (CTA) button are usually the main focus for the companies website. These buttons are how they convert sales.
As such, standard practice tells us that CTA buttons should be placed at the top so that it's one of the first things the user notices.
"Oh hey, that's where I press if I want to buy the product, good to know,...