They’re often not the cheapest

No, but they're consistently reasonably close to that. I'm not likely to pay drastically more on Amazon than elsewhere.

What they do have is a very large selection, which is a major benefit; I don't want to deal with eight different retailers and eight different orders when I can just hit Amazon.

At a lesser scale, this is what made department stores do so well.

https://en.wikipedia.org/wiki/Department_store

A department store is a retail establishment offering a wide range of consumer goods in different areas of the store, each area ("department") specializing in a product category. In modern major cities, the department store made a dramatic appearance in the middle of the 19th century, and permanently reshaped shopping habits, and the definition of service and luxury.

Amazon's basically done the same, writ large. They can offer pretty much anything you want from a consumer standpoint at a single location.