The way Amazon works, I suspect the majority of the scuba gear they sell actually still has a scuba shop of some kind behind it. But anyway....
I agree that it would be smart for every shop to also sell online. However, the costs of doing so are not nearly as cheap as some people seem to think. Someone upstream talked about paying a high school kid $100 to setup an online store for a shop. That is not even REMOTELY a reflection of the real cost of doing so.
Plus, the reality is that implementing and running a good quality and successful online store requires expertise and skills that most dive shop owners not only don't have but mostly it's a case of they don't even know what they don't know. So, to put up their online store they would have to pay someone - someone expensive, if they are going to really get good results. I do this kind of thing for a living. It's not really different than scuba instruction. You can pay somebody that is cheap, but you're almost certain to get what you pay for, so to speak.
Bottom line: Most scuba shops simply do not have the resources to put up a competitive online shop.