I think Amazon might have induced this problem somewhat. Most retail stores have just stopped stocking niche items because people just buy them from amazon and it frees up a lot of floorspace they could use to stock high sales volume items instead.

But that's just the basic physics of mail-order shopping, it isn't anything specific to Amazon.

Every general store in every town everywhere can't possibly stock millions of individual products. When some item has a potential customer base of a few tens of thousands of people in a country of hundreds of millions, most stores would never get a single sale for that item. But when you can serve the entire country at once via the mail, having a single centralized warehouse specializing in that niche category of items is quite viable.

The only difference with Amazon is that you are buying from a single website, rather than going to the website of a small webshop specializing in your flavour of niche items. And the drawback is that Amazon has zero quality control, so rather than a store where you can browse for known-good curated high-quality niche items you will instead be drowned by piles of absolute garbage - with the occasional decent item hidden in between.

Sure, Amazon might beat your local store, but that doesn't mean it is good for buying niche items.

Basically the only things I don't buy from Amazon is groceries and large kitchen appliances, and it's 90% due to the convenience.

Yep, I've always been a traveller of the long tail.