You also have to look at the success and failure rates of those tests. Most tests reveal no problems, which implies periodic sampling is plenty to handle the rare problems that crop up. If we found more problems, we would demand more testing, but increased testing is pointless if there is no problem to be found. In reverse, if the tests are not specific enough, they can cause issues when you over test due to false positives on the tests.

Indeed, you'll see that if a water test comes back positive, there will be multiple retests and a much greater rate of testing until the problem is abated, at least at my local drinking water board.