No different than when you go to the store and the store's brand products are at eye level and the name brands are lower on the shelf.
The lack of competition in US (Walmart and Target took e-commerce seriously only recently) let Amazon take almost 50% of e-commerce. But hopefully Ebay, Walmart, Target and Costco would add more competition.
Painting Amazon like this only online market that we cannot escape is a bit misleading. Amazon has a long way to go to beat the asian e-commerce, and now has domestically more competition with Walmart, Target and Costco.
Amazon is barely holding on ~44% of the US eCommerce market.