[coldtea]

"Historically" is the keyword here. Evolutionary timespans are much much larger than recorded history and human culture as we know it.

[toasterlovin]

Nobody's saying that grandparents being alive are a prerequisite for grandchildren. But anybody with kids will tell you that grandparents are tremendously helpful in raising children.

Evolution is statistical in nature, not binary. Grandparents just have to be useful in raising grandchildren for them to be favored by natural selection. They don't need to be strictly necessary.