It's not the preventable diseases that are the most costly to treat. But regardless of semantics, I know what you mean. The reason is that the consequences of guaranteeing everyone enough food are different than the consequences of guaranteeing everyone top-of-the-line (i.e. doing everything we can to keep sick people alive) healthcare.

Food has become so cheap that almost everyone can afford enough to eat without direct subsidies (e.g. indirect subsidies, like welfare payments, are enough). So such an understanding doesn't result in the creation of pervasive government programs that utterly dominate the market for food and inflict it with the cost disease.