Healthcare is not irrelevant, that is a ridiculous statement. Perhaps you mean healthcare spending, but even that is only supported by a few outliers like the US.

Lifespan statistics are heavily influenced by early deaths. Drugs and violence take many more lives in the Western Hemisphere than in Singapore. This is enough to explain a large portion of it. Especially during this opioid epidemic.

I'd argue that diet is significantly worse throughout the Western Hemisphere as well, but nutrition is a difficult science.