Lets say 3 in 100 reviews are good. And you want those 3 to come back and review the next product. You are even willing to pay them something. Is there a guarantee that they have interest and will show up, and if they do, is there a guarantee that they produce a useful review of the same standard the second time around?
Thats just one simplified reason about the dynamics of this process, that is going to block everyone from getting things for free.
Its easy to react, to push to dismantle it and you just end up with YouTube style mega mass of comments under every vid where the best comments are guaranteed to get buried unless they pander to the will of the herd.