Amazon's incentives lie in their buyers buying a product, and not in their buyers buying "good" products. That's why they are not motivated to guide the consumers towards better products by sorting out this mess of a review system. Now a fun little exercise would be to consider what if Amazon indeed cared it? I'd imagine that it'd:

1. Post statistic of how much that product was bought vs how often it was returned within a timeframe.

2. Put greater emphasis on the reviews of longterm accounts with significant purchase history.

3. Not allow any accounts that have not bought the product to post a review at all.

4. Punish products heavily if evidences of review tampering were found.

And so forth