They've had a long time to figure that out. It's been an issue for a while. They've just neglected to do anything about it.
It doesn't take a genius to look at all the reviews that some products have and realize that they're fake
It doesn't take a genius to look at all the reviews that some products have and realize that they're fake
It's not neglect, it's a very hard problem, as is any problem where you are trying to make an inference with limited information controlled by an adversary. Amazon could spend billions on incredibly sophisticated fake-review-detection algorithms, but they still couldn't stop sellers finding real amazon customers and paying them to buy and review their products.