This is a good point and your comment below ( https://news.ycombinator.com/item?id=37768419 ) highlights it well. What people don't realize is that this is in the interest of the consumer. I used to work at Amazon (although not on any of these teams) so that's probably why I know that "see all buying options" means the price is not competitive. As a consumer I value this.

Amazon scours the web to make sure their products are competitively priced. So if a Western Digital external hard drive is $99 on NewEgg, then Amazon wants to sell it for $99 or lower. If it's a 1P product (i.e., Amazon is the seller), they can easily price match in real time. But if it is a 3P (third party seller) product then Amazon cannot take that action unilaterally take that action since it is the seller setting the price. So the best they can do to protect their customer in real time when their crawlers detect a lower price elsewhere is to hide the buy box behind the "see all buying options" button.

I'm sure when this feature was rolled out, Amazon's sales must have dropped. They are taking that short term hit in order to protect customer trust in the long term.