They are absolutely not close to a monopoly. They have tons of competitors. Walmart has basically the same exact service online. You can get your crafts on Etsy. You can get your tech supplies on a ton of different websites. You can get all your hardware supplies at homedepot (online and b&m) and the list goes on and on and on. If Amazon disappeared tomorrow, you could get literally everything the minute it happened on many other places. You can even get all the same chinese knockoffs right from the source, aliexpress.
A monopoly means you are _forced_ to patronize them. I know a lot of people who have chosen specifically not to shop at Amazon and they don't have a problem getting what they are looking for.
They have a monopoly on offering two day shipping and incredibly easy returns and an extremely reasonable price, that's why people use them.