Many of the students in the class got through those prerequisites with, say, 70% on the final exam. That means they have a very tenuous grasp on much of the content, and so teaching with
at least some overlap with the prerequisite is useful.
The prerequisites may have been taken 6-12 months earlier. Without constant use, a lot of the concepts that were learned in the prerequisites will be lost over that time period. Many classes will indeed start with the first lesson or two as a revision of the prerequisites for this reason. It possibly annoys those who have a firm grasp, but not everybody learns mathematics (or language, or anything else) the same way.
