Might be as simple as cost/effect in resource-constrained environment. Inflamation uses up resources. When we were hunter-gatherers and had to survive ice ages

Might be as simple as cost/effect in resource-constrained environment. Inflamation uses up resources. When we were hunter-gatherers and had to survive ice ages - it wasn't a good idea to waste calories and vitamins just in case. Better for 3 people out of 30 to die of flu than for all 30 to starve. Nowadays the optimal trade-off might be completely different.