It makes total sense to go to Amazon to buy Echo, eero, Ring, Blink products, why wouldnt I buy them from their manufacturer?
They do by soft power -- leveraging their position in the market to make the alternatives completely unprofitable, thus literally preventing you from selling on other platforms.
Are you describing predatory pricing? Where they sell at a loss to put competition out of business or to grow faster? Or are you trying to say its not profitable to sell on ebay or shopify instead of amazon? Amazon using its scale to clone products and sell them with low margins, in an effort to prevent competition from springing up is anti-competitive, but its not them being a monopoly.
All I mean is the e-commerce space is becoming increasingly "monopolar" due to Amazon's activities.
The stores pushing their brand helps with the paradox of choice, comparison shopping, recommendations and reivews, and blame when something goes wrong. I expect them to do the work of selecting the best product and standing behind it. Its department store scale boutiques, store brand product curation (basically white label rebranding, maybe custom runs off the factory lines) is a feature, not a bug.
Amazon is doing a very similar thing by buying brands. Hopefully someday there is more to Alexa than voice, and there is a single something that allows you to see basic status and config of all your amazon products and services, whether they be Echo, Fire, Ring, Eero, Twitch etc.
I actually do go to Amazon to buy Kirkland. The nearest Costco is far enough away and not sufficiently cheaper than the Walmart supercenter a mile from my place to justify a Costco membership.
Best Buy is actually a good place to buy Amazon devices if you have a nearby Best Buy. They price match so the price is generally the same (and better if it happens to be a time when Best Buy is having a sale and Amazon is not), and you get the thing right away. Oh, and Best Buy takes Apple Pay and cash.