It's the all you can eat buffet effect. Pay the price and don't have to worry about shipping, can watch (some) streaming without having to worry about paying, and whatever else they decide to roll into their monopoly black hole today.

Sure, if you do a full accounting of costs you may win or lose, but fundamentally people are paying for simplicity. Because almost everyone is lazy, or too busy, or too afraid of random scammers, or whatever, and they played their cards right to become the Sears Catalog from the 19th century in the 21st century.

edit - and one thing that helped them get there is the return policy, so if you get one of those scam sellers, or they sent you wrong crap, opened crap, or just plain everyday crap, you press a couple buttons, maybe drop something off at a UPS store, and problem solved. That definitely shields them from the fallout from their endless listings from sellers like QWERTY123 and ZXCVBN789, and provides an advantage over any other online ordering that doesn't have the same massive advantage of scale.