Really? I had no idea! /s

Seriously, though... just because there are other vendors that sell online does mean that each and every thing someone may want/need is available from other vendors online. Amazon has spent decades forcing competitors either out of business or to work through Amazon. They also leverage volume and loss leaders to drastically undercut prices of competitors they can't eliminate.

Even if you can find someone else that has what you're looking for and are willing to pay more (and for the record, I absolutely pay extra to avoid Amazon whenever possible), there are a lot of small businesses that provide even shittier service (or are outright scams) than Amazon. You may or may not actually get what you ordered, and if something is wrong with it, good luck getting a replacement or refund short of a formal dispute with your bank. Many require credit cards for online transactions, and you have no idea how they're handling that data. Plenty exist that store CC info on local servers with nonexistent security precautions.

"Just shop online elsewhere" is just as lazy and undeservedly self-righteous as assuming everyone can just walk to a physical store to buy whatever they want.