Fake reviews are usually positive. That is just advertising for Amazon which unlike normal advertising HIDES the fact that it is paid-for advertising. Amazon benefits when there is fake positive advertising about stuff sold through them.

Now it is possible that Amazon actually does a good job of stopping fake negative advertisement on its site. Good for them. What would stop them? I don't think there are any laws against putting more money into stopping fraud which does not benefit you than fraud which does.