Healthcare doesn't stop people from spreading colds. You're not actually reducing externalities with most categories of care. And if there were no police you would see a lot more theft.

Anyway, you can put it in cost-benefit terms, but you have to include a strong freedom factor. A government that takes 2% of GDP to keep public order is far better than not doing that. But taking a very large fraction of income to provide health care? That is an enormous cost to both spending and freedom, and reasonable people can see those costs as higher or lower than the benefits.