The difference being is that all the companies spending more than Amazon can recoup their costs in the price of their product. Not much competition.
Amazon has too much competition, and must keep their prices low in order to survive. They cannot recoup their R&D costs, unlike Pfizer, Microsoft, Oracle, etc.
The second factor is what is exactly included in the Amazon R&D percentage?
Are their airplanes, new warehouses, etc included in this percentage?