>> Amazon has perhaps 1% of the US retail market by value.
This isn't surprising considering there isn't much competition (in terms of brick/mortar) in most of the US.
Electronics? Best Buy
Books? Barnes and Noble
DVD's? Best Buy
Video Games? Game Stop, Best Buy
Without much competition in most categories, it's not surprising most people opt for online shopping instead of going to the same place, paying much more and having a crappy in store experience.
This isn't to say other categories probably have much better local options. For myself, I've rarely purchased bike parts on the internet unless I'm completely sure its a good deal. My local bike shops tend to have what I want, or they can special order it for me and its still pretty close to cost.