Have all your tanks been setup for awhile?
@omykiss001's theory about older tanks having better nutrient export and that, in turn, keeps ich within manageable levels is interesting:
https://www.reef2reef.com/threads/ich-eradication-vs-ich-management.188775/page-4#post-2662497
Especially when you consider most of those who ask for help in the fish disease forum are noobs with very new setups. Of course, you could also say they just aren't experienced enough to manage their tank, and ich, properly.