It's not that hard - look at systems like Fakespot that could automatically verify reviewer credibility (without even access to the internal data that Amazon holds).

Or just offer systems for buyers to report suppliers doing exactly this - buying reviews.

Or honeypot it, and just put some fake addresses on sites that sell reviews and then ban companies that send you stuff.

I mean there are loads of solutions, but all of them involve Amazon taking down 5 star reviews that help them sell crap, so the incentives are obviously conflicting here.