They have the resources. What they lack is the proper incentive. As long as fake reviews don't completely destroy the customer experience to the point that people forego any other advantages (mostly convenience) of shopping there, Amazon will continue to reap the short-term benefit of more apparent variety, more apparent quality, and more sales. Any effort beyond keeping the fakery just short of destroying the brand isn't worth it to them.
I'd venture further and claim that it's in Amazon's long-term interest to drive down the perceived brand value of third-party suppliers through its distribution channel.
Costs are lowered and the value of Amazon's brand is relatively increased. Similar to stocking own-brand product in a supermarket. This weakens the negotiating position of big name brands.
