Because food is predictable, available, and cheap. Healthcare is unpredictable; you don't know when you'll need it and you hope you never do, but if you do, it could completely bankrupt you and since your life could be on the line you'll do anything to get it.

Also unlike food, since we have a massive insurance industry involved to spread the risk of unpredictable and often massive expenses, we've actually created a system that incentivises people to hurt other people by denying claims. Not everyone believes the invisible hand of the market is the correct solution to every problem; I'm all for socialized healthcare.