We can only assume that every cow we milk is Kosher based on rov when we milk a single cow then drink that milk. When we combine the milk from hundreds of thousands of cows and we know that more than 1/60 of all cows are Treifos then rov should not work.
The answer given (by Rav Elyashiv, Rav Chaim Kanievsky, and others) is from a Rosh quoting the Raavad that if something is Yavesh Beyavesh Veachar Kach Lach Belach (first it is a non-Lach Taarvoes and then becomes mixed in Lach Belach) then you only need Rov. Since each cow is still milked individually it starts out as a Ta’aroves of Yavesh Beyavesh, at which point it becomes Muttar, and then does not become Assur once all the buckets (or wherever wach cow’s milk goes first) are combined, even though it is now Lach Belach.