Well, you might think that. Did you consider that you might be wrong and maybe Amazon actually has lot of data that you don't and they know it better?

Just a couple of examples.

I might have bought a hard disk drive a couple of days ago and realize that I actually want more so I can do backups (or I want to build a cute HDD tower with a Raspberry Pi).

I might have bought ear plugs for home, and decide a week later that it would be nice to have those at work, too.

I could have bought zip ties, only to realize that I actually want more in different sizes and colors.

I bought a pijama, it was delivered and I want to buy 3 more of those, so I can throw out my old ugly pijamas and not worry about buying new pijamas for the next 5 years.

I got headphones but they aren't quite perfect. I didn't issue a refund yet, but I know I'd eventually send them back and want to buy a similar headphone asap.

Where do you see those ads?

The most prominent ads I see on amazon is when I search for literally anything, the top results are often whoever paid Amazon the most to be the top results. Sometimes I will literally search for a brand by name and some other brand will be shown before it. This kind of race to the bottom is something I've heard a lot of companies complain about, companies literally need to pay Amazon for the ads on their own terms to outbid competitors.

My guess is that they make sure that you don't doubt your purchase. You could send it back if a competitor gets into your head. Additionally, if you don't doubt, you become loyal to the brand and you will recommend it in your network.