Because Amazon benefits from massive economies of scale, and because Amazon has nearly-unlimited cash to push competition out of the market?
How could a regular webshop possibly compete with a behemoth which has warehouses in literally every major city, which has its own dedicated fleet of delivery vehicles, and which has its own fleet of cargo planes? When you've got a single warehouse and have to rely on DHL/USPS/FedEx, you will never beat them on delivery time when Amazon is actually trying: Amazon can trivially offer same-day delivery and eat the cost until you go out of business.
And don't forget: Amazon has an even bigger margin. If they were to compete on price, they would easily win. But they aren't competing on price because they know they have a quasi-monopoly, so they can just enjoy the extra profit.
How could a regular webshop possibly compete with a behemoth which has warehouses in literally every major city, which has its own dedicated fleet of delivery vehicles, and which has its own fleet of cargo planes? When you've got a single warehouse and have to rely on DHL/USPS/FedEx, you will never beat them on delivery time when Amazon is actually trying: Amazon can trivially offer same-day delivery and eat the cost until you go out of business.
And don't forget: Amazon has an even bigger margin. If they were to compete on price, they would easily win. But they aren't competing on price because they know they have a quasi-monopoly, so they can just enjoy the extra profit.