1)Fake reviews are an actual hard problem to solve. Good reviews can really make or break your company so the incentive to game the systems is quite high.
Positive reviews for your own company, negative reviews for your competitor, or cunningly, clearly fake positive reviews for competitors to get them in trouble with the review site.
2)Incentives lead these review-centric companies to some bad outcomes (or at least the lack of trust by consumers). Interestingly, companies with better revenue models outside of the reviews may end up building the most trust among consumers.
I would think Google is very well positioned here. Both because they don't need to get the $$ from scammy tactics and because their user data can help them find fake reviews much more easily. Did user XXX use Google Maps to get directions to the location? Did user XXX get an emailed receipt from that company?