Lack of available reviews on scientific papers It is common practice in computer science to have papers peer-reviewed, however this is often not made public and after the paper has been published it is hard to find ANY reviews on the paper. There will always be mistakes in scientific papers and if someone points out the flaws in a simple manner in a review it could save everyone a great deal of time. Some things are of course too complex or subjective, to be put into a single paragraph, but there are plenty of things that can be pointed out.

My question is why are there no reviews included on sites like ACM or IEEE? Papers sometimes have thousands of citations, but not a single comment or review has been linked. Or are there simply no reviews and comments available? Does the conversation and discussion take place somewhere else?
Usually the issues pointed out in a review have been corrected by the time the manuscript has been published, so old reviews wouldn't really be relevant to the online version. 

Critiques of published articles are sometimes prepared, if one thinks the issue is significant enough; they're usually published as "Comments" or "Responses" to the article in question. However, since they're normally also peer-reviewed, they won't simply be appended to the article, but instead have their own independent status.

As for the reason why more articles don't have reviews and commentary associated with them—it takes a lot of time to do so, and that's time that can be used to make progress in one's own research. And there isn't much incentive right now for such efforts.