Put more simply, Amazon's warehouses are filled with knockoff RAM chips, memory cards, and other such hard-to-identify components. Amazon isn't doing this deliberately, but your odds of getting bad RAM, HDD, or flash memory is really quite high.
It's been a while since I worked there - when I was there there was a big problem with inventory mixing. Third party merchants selling on Amazon would routinely ship items to Amazon warehouses to take advantage of Amazon's fulfillment process (this is known as Fulfillment By Amazon), and a huge amount of gray market goods were able to get into warehouses this way. Amazon's own supply chain integrity is, by itself, actually pretty good.
For a long time Amazon allowed these third party-sourced items to mix freely with the legitimate items they purchased themselves, and if you ordered the item (regardless of which merchant you ordered from) you'd be drawing from this shared pool. It was frustrating, and I'm not sure if they've fixed it yet.
A big one is Newegg. Most things I'll search for on Amazon first (or sometimes Google if I suspect there might be review content somewhere else). But computer parts are always cleanest on Newegg, and it's not even close. Their selection is better, and their parametrized search is far more comprehensive.
Now, computer parts are a shrinking market, and Amazon isn't hurting because of this. But I think the broader point is that while Amazon does things really well, they don't do everything perfectly. And customers (like me) are fickle, and will happily kick them to the curb if something better comes along.
Basically Amazon, for all their power, are still selling what are essentially commoditized products. And the barrier to entry there is very low for a player that can actually compete on price or quality. Whether Google is such a player is an open question.