A Prime membership means you are paying for shipping. A standard membership is $12.99/month or $119/year. That means all of the “members” are collectively paying a monthly/yearly flat rate shipping rate that includes access to movies, shows, music, and books. If you rarely utilize those (or not at all) then you are essentially paying for your, and everyone else’s, shipping at a deep discount. If you rarely order from Amazon then you are literally paying for everyone else’s shipments.
That business model, combined with their drivers that essentially do the same job for a fraction of the cost, and the fact that both UPS and FedEx helped Amazon jumpstart that model by providing logistical help and ridiculously deep discounts, and the fact that there are thousands and thousands of Amazon zombies that are too lazy to do a few minutes of research to find a better price somewhere else, is exactly why Amazon is taking over as the biggest delivery company.