Amazon exists to increase shareholder profits. The reviews are not exempt from that. They exist to encourage customers to buy things and to increase sales volume. When Amazon was up and coming against a lot of other competitors it was relatively more important that reviews provide a good signal for customers. Now that they are more hegemonic, that is less important relative to facilitating increased sale volume, and there is also more incentive for sellers to fake them.

Everyone's incentives are cleanly aligned for the short-medium term, except for those of consumers and workers.