Because people have only so much cognitive load, adding to that to the list of things they would have to do on a daily basis is simply daunting.
This just seems like another way to rip off people who don't know better and are disadvantaged; imagine the haggling difference at the poor shops vs the rich shops. The rich person may be able to afford the extra cash and you may think the poor person would be incentive to haggle harder, but the rich can also afford to leave to go to another shop with the car they own instead of spend another hour on the bus to get to another place. They can also afford to be informed of the prices.
It's a fair point certainly, in regards to poor shoppers. Most American consumers aren't poor, have Internet access and can easily compare prices, and most of Amazon's customers also are not poor.
In fact today it's extraordinarily trivial for the average person in the US (Amazon's core demographic still) to price compare. It takes seconds via Google or directly via Amazon.com or Walmart.com etc.
I don't believe price comparison is daunting or overload, for the same people capable of consuming hundreds of status updates on social networks ever day. In fact, the history of the American consumer indicates price comparison is extremely easy and natural, said shopper has been price comparing and seeking out deals since the birth of modern shopping.
This just seems like another way to rip off people who don't know better and are disadvantaged; imagine the haggling difference at the poor shops vs the rich shops. The rich person may be able to afford the extra cash and you may think the poor person would be incentive to haggle harder, but the rich can also afford to leave to go to another shop with the car they own instead of spend another hour on the bus to get to another place. They can also afford to be informed of the prices.