Sure, it would have helped homo sapiens even more if more of their children had lived to childbearing ages (if, say, they'd discovered antibiotics a few millenia earlier or evolved more natural immunities). But the infant survival rates back then were enough to ensure the species survived, just like the smaller fraction of the population struck down by skin cancer every year wasn't enough to put evolutionary pressure on us to develop resistance to it or develop sunlight avoidance traits. Evolution doesn't guarantee that every possible beneficial adaptation takes place.