Some people actually can't, or are simply better off without. And many don't realise they're in that group. One part of the reason is the unavoidable fact that "milk" is not a singular, definable substance. It varies wildly, depending on locality.

Eg look up permissible faecal, urine and pus levels where you are, versus other areas. Also, check out differences in store and hold requirements. Finally look at what substances cattle are allowed to be injected, or fed with. Each varies greatly.