I think wikipedia, HN, etc. are less suseptible because people don't use those sites as heavily to make purchasing decisions. Any community that helps with purchasing will be targeted by those interested in sales who figure out how to meet the "high bar" no matter what that may be. Even purchase verification isn't high enough these days (see recent unsolicited package scams such as the seeds from China).
I agree with you: It's a numbers game. There is a cost associated with driving up good reviews for a (bad) product. A website such as this has to cause higher costs for fake reviews than what they can reap via it.
However, at some point the whole review process might become unusable for legitimate users, resulting in too few reviews, rendering the whole endeavour futile.
E.g. if a shitty product earns a seller 20US$, and they expect to sell exactly 100 more over a site like this, then they can easily invest 1000 US$ into trying to make their product seem good, even if it isn't.
Thinking of this, by that reasoning a non-negligible part of the high cost of good quality products might also stem from the fact that advertising genuinely good quality must be expensive like hell (I guess).
