JWC, MidlifeAspie and Deaconblues BOTH gave valid answers. If the market conditions are different, then something CAN make economic sense in one situation and not another. Economic sense and economic reasoning is sensitive to issues of incentives, # of providers, cost of gaining information, bargaining position, etc.
The big principle of economics is that incentives matter, however, in various fields we CAN get different results. For instance, increases in wages CAN lead to less hours worked, which is unlike other fields of economics. Reason? The number of workers possible is relatively constant, and they always face a trade-off between work and leisure. While some people if they get twice as much per hour might work more hours, a lot will want to work less and leisure more.
It may be true that single-payer medical care is not justified, but the comparison between it and groceries is not sensible. Medicine does have more information issues than groceries do, as every consumer is going to have a vague idea what their Cheezits will do for them, but few will have any idea on what their hip surgery or Glycolopaxinil will do. Even further, Cheezits are not horribly expensive, but surgeons are, leading to concerns on consumer credit. The providers of medical health is an issue as one cannot really shop around for medicine well. Bargaining is an issue, especially given that people in need of immediate aid CANNOT bargain. Incentives might be more perverse as your doctor might recommend Glycolopaxinil to you wrongly due to pay-offs from a pharmaceutical, but that issue is not even similar when talking about Cheez-its. Finally, even if the economic reasoning is the same, the ethical reasoning is not. People need both medical care and groceries to survive, but nobody needs grocery insurance as grocery costs are relatively constant, while medical costs are variable and can be extravagant causing real human interest issues in the medical care market's outcomes.
I am not saying that one cannot argue against single-payer healthcare, but.... you've gotta do better.