Random internet individuals: "But they could just..."
Like so many things/problems - they all seem easy/obvious until you actually TRY TO DO IT, at which point you realize why all of the other very smart people haven't solved it yet.
I put it in the same bucket as fake accounts on Twitter. My guess is that Twitter can correlate many accounts that do nothing but troll and shill for particular factions, and probably has a registry of these accounts, but they also lose MAUs when they nuke accounts. They have an incentive to wait until user growth comes in stronger than expected before they ban lots of active users.
For me personally, the biggest weakness of Amazon is commingling of inventory. I never buy things like lubricants and bearings from Amazon, even if the seller is an authorized distributor, because there's no guarantee I will avoid knockoffs from a different seller. I'd rather buy from Walmart or pay more for a specialty retailer to ship me the product, because Amazon inventory is often mixed with cheap knockoff garbage.