Of course it's subjective. Why is that relevant? Math teachers are teaching a course with certain prerequisites, so in that context "trivial" means something like "it should be easy/obvious for people with the appropriate mathematical background for this class."
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.
I've never taken a math course that didn't contain some overlap. But you simply can't realistically teach everything from every prerequisite in every class. There may always be some students who snuck through a prerequisite without learning or retaining the info. That's what office hours are for.
