Money. They only care about short term sales. Amazon doesn't care about how much you trust amazon... for now. Once they've lost everyone's trust then they're start to care.
As far as I know (and in my experience so far, e.g. reflected in very lenient return policy) customer trust has been their highest priority from their beginnings, which is one of the main things that made them successful in the early web in the end.
They have been risking it by their issues with counterfeit products and fake reviews only in the most recent years, but I would be surprised if they had no engineers who are thinking about ways to solve these problems.
