@sinbad long story short, the engineers get paid a bonus for each leak that's over a particular threshold. So those are the leaks they go to. And they know exactly where they are, because their fixed aren't designed to last forever. Why would you fix something for good if you're going to be paid a bonus each time you go back to fix it again?
So the big clusters of small leaks that lose tonnes of water get ignored, and the bigger individual leaks get patched up on an almost fixed rotation.