Simply because people tend to take a lot less time/effort to review edits than when actually editing the post themselves. When all you need to do is click one button ("approve") it just becomes too tempting to click it without putting in the time and effort to read the post through thoroughly as well as to look at exactly what the edit is suggesting to verify that it's a proper edit. Too many people have been (based on lots of historical evidence and data, not just theories) just approving everything, or if not that extreme at least making decisions at just a quick skim over the edit without really giving it the time that it needs.
In order to deal with these problematic reviewers one effective means (among other features of the review system, such as a daily review limit and audits) is to require more than one person to review posts. (And more than 2 in the case of SO.) Among other things, it means that if people pay attention to how others vote they will notice when reviewers aren't doing their job, or when a post is improperly reviewed (so that they can go back and fix it).
Ideally, yes, this system wouldn't be needed and there would only need to be one reviewer per post. Alas that simply can't be the case due to the [rather large] number of low quality reviewers using the system.