Why do reviewers summarize a promotion application?
In a typical letter, the overlap with the CV is just a small portion of the letter, so I don't see why it's a problem.  (If it accounts for most of the letter, then something is wrong.)  You're right that it's not logically necessary in a world of people with perfect memory, but it helps remind people of the basic facts and puts the subjective comments in context.  Only a terrible letter would announce "Smith has published seven papers" and then never say anything more about the papers.  It's much more reasonable to lay out the facts and then go on to offer more detailed and evaluative commentary.

There are also issues that are specific to letters of recommendation.  For example, letters of recommendation are often updated over time.  Perhaps the first letter I write for a graduate student is part of a fellowship application.  When she graduates and applies for postdocs, I substantially edit the letter but don't necessarily rewrite it from scratch, and the same document may even last until her tenure case (of course it evolves dramatically over time - by the end four or five substantial changes have been made, and almost none of the original letter remains).  Now one purpose of reciting basic facts is to make it perfectly clear that my judgment really is based on her current accomplishments, and not just thoughtlessly recycled from a previous letter based on older information.  Of course this is far from the only way to show that, but it can't hurt and might sometimes help.