They have created solid economies of scale that others can't match. Together with the massive amount of customer data they are in a position where they can optimize their prices and respond and react in almost real time to get optimal customer retention.
The long term goal is dynamic pricing for the perfect price discrimination (personal pricing). Detect when the customer is tired or stressed and show higher prices. Detect price conscious customers and show them lower prices. Maybe Alexa can trained detect the mood from the tone of voice and exploit that.