Well, for the most part, they don't really do a very good job. And the writing exercises they conduct get a little old....
For the best writers, these "exercises" are analogous to working through a solo or a composition for a musician. Writing is an art form itself, a difficult one, and just because it takes music criticism as its jumping-off point does not mean it isn't valid on its own terms. There are bad writers out there just like there are bad painters and bad musicians, but hopefully with time, those writers will become very, very good or even great.
I respect your admiration for reviewers.
Are you one?