More often than not it's because of the conditions the animals are kept in.
For instance, in order for a cow to produce milk constantly they must be calving. That means they are repeatedly artificially inseminated (some hardline vegans would call this rape) and the calves are then usually kept for veal. Additionally, milk may contain hormones, antibiotics, puss and blood. The values vary country to country based on what is deemed as "acceptable" but I believe the UK allows 150,000-200,000 animal cells per ml of milk.
For instance, in order for a cow to produce milk constantly they must be calving. That means they are repeatedly artificially inseminated (some hardline vegans would call this rape) and the calves are then usually kept for veal. Additionally, milk may contain hormones, antibiotics, puss and blood. The values vary country to country based on what is deemed as "acceptable" but I believe the UK allows 150,000-200,000 animal cells per ml of milk.