Same question applies to Twitter. Regularly we see researchers uncovering evidence of fake accounts and bot networks pushing spam, conspiracy theories, and misinformation (https://www.npr.org/sections/coronavirus-live-updates/2020/0...) while Twitter struggles to identify the fakes.
ETA: Regarding comments along the lines of, "it's in their best interest to let fake reviews continue as it boosts Amazon's sales."
I think this argument is getting a bit old, as Amazon's growing reputation for fake reviews (and fake products) is turning off even mainstream shoppers and giving a huge boost to big & small competitors.
Amazon now has a very big financial incentive to move beyond whack-a-mole and truly tackle these problems, which requires A) doing at least as good as Fakespot at identifying the bad actors and B) implementing technologies and policies that discourage buying or posting fake reviews.