I expect this is why the quantity is more up-front now. I bet it is pretty unusual for anybody to buy multiple copies of a given book at once, so defaulting to one and giving the opportunity to adjust later made sense. Now that they sell many things that are probably bought in multiple quantities (say, socks), customers are probably having to adjust the quantities more often.
All very well, but sometimes I wish they would spend a bit more time on their wishlists - not much has changed since the beginning and some more effort in this area is overdue by now.
"Back then, ... Amazon, for the most part, sold books."
I just consider them a generic shopping mall now.