Because you might not poison only yourself. You might become a breeding reactor for a new strain that more easily spreads between humans, affecting those who ar

Because you might not poison only yourself. You might become a breeding reactor for a new strain that more easily spreads between humans, affecting those who aren't drinking risky milk. Same reason why people care about others not getting vaccinations for easily communicable diseases that can kill or debilitate. If they were the only ones who could be affected by that decision, fine, do what you want. But you put others at risk, a risk they may not be happy to need to burden themselves with.