How to find out who were the reviewers of an article? I understand the concept of (peer-)reviewing as helpful to guarantee a good quality result. Clearly it makes sense that journal articles are reviewed by someone before publication. Yet, what I am still unclear about is who the reviewers are? The focus of this question is not who can peer-review articles, as I am not interested in who qualifies for being a reviewer, but rather about how to find out about the actual people having been involved in the review process?

It disappoints me to not be furnished with a list of the reviewers as it would help me tell if the article is likely to be well-reviewed or not. In academia, where reputation is paramount, it would seem imperfect if the people behind the reviews are kept secret. Yet I have not yet encountered a list of reviewers for a specific article and the best place to put this information seems to be with the article itself. 

Another worthwhile information connected to it would be the number of reviewers. After all the more people investing time into a review of some contribtion the higher I assume to be the chances that flaws and problems become corrected and again the more interesting the contribution may become. Since unfortunately there is an excess of publications from people needing to make a career and reading through all of those articles constitutes an obstacle more than an accelaration of the scientific progress.
Although @xLeitix answer is spot-on, I think you are missing the main point. Anonymity is directly linked to any democratic process. Likewise, your election vote is anonymous. In this sense, a review is just a vote of confidence for the reviewed article and therefore it has to be and remain anonymous for a more objective opinion. 

Another point is that anonymity in reviews not only protects the reviewers against repercussions but also protects against nepotism and mutual exchange of favors. Moreover, anonymity also ensures that all reviews are (almost) treated equal. So, a favorable, short review from a professor (who just said ACCEPT because he might personally know the authors) might count less than an informed, in-depth review even if that review comes from a PHD student. So, although the editor knows the reviewers, one reviewer cannot argue / discard with the other reviewer based on their individual status. In this sense, anonymity also protects the reviewed, since if the paper is actually good, it is more probable to be reviewed based on its merits then the authors' public relations.