I think it's because eBay, since it's founding, has had a scam problem that it's been actively trying to address. They aren't perfect, but they have certainly been putting in multiple measures to increase trust.

Amazon, on the other hand, has nearly done the opposite. They don't do anything about scammers and have created an environment where even when you buy something directly from amazon you might get a knock off. You can't trust anything there.

Everyone knows the amazon reviews are garbage, even amazon. Yet they keep them because having 1000 garbage reviews looks better than having 10 legitimate reviews (and many of them negative).

Amazon has optimized for moving any and everything.