Well he most obvious reason and most likely the actual reason is game developers don't want to have to deal with the bullshit of the moral implications of infanticide.
But there are really two questions:
Why IS it not?
Why SHOULD it not?
The question is like an elephant in the room, it is quite clear by the precedent set by both games and tv shows and films, you can get away with senselessly killing hundreds of innocent people, even in a film like HEAT (yes, I always write that film's name in upper case... don't know why) in the intense Bank Shootout Bob De Niro's character actually shoots and kills a mother and the film doesn't skip a beat and he remains a sympathetic character. Yet Sizemore's character takes a little girl hostage and when Pachino's character kills him with a carefully placed shot (yes, I really have forgotten all the characters names, boy, I thought I was a fan) there is no remorse for sizemore.
What does it say about us that we hardly blink at innocent adults being killed, we see it on TV, both fictionally and in real life, we accept it. We watch it as entertainment.
What does this say about our society?
What does this say about our species?
We should be just as shocked by adults being killed as children being killed. Yet we aren't. they're all fellow humans. They both have so much to contribute to society.
Maybe... maybe it is because as a society we don't have the common social connection we used to have.
All we have left are our animal instincts, to spare children because they are no threat. But anyone else, an adult, they are fair game in this rat race.
I realise that people only get emotive about adults being killed when there is empathy... a connection. Things like "For each person who died, they were someone's brother, someone's son, someone's husband" That is why I can hardly care about running over pedestrians in GTA, not matter how realistically their bodies go flying, there is no way I can really interact with them. I mean if you have ever played GTA4 the character models are OK, but they still act like androids.
This is seen in war accounts, especially pilots, they see people get blown to pieces but they admit they are surprised they are not shocked by it.
I think about deaths in games. Those I care about were those the developer had ensured I had created an emotional bond with through careful characterisation and interaction them. In fact in games at least I have cared more about the in game deaths of non-human characters (those that can't even speak) than realistically human characters who are un-relate-able. Because when it comes down to it humanity and species loyalty doesn't count for anything.
10,000 innocent civilians were killed in some civil war = meh
but if your pet dog dies...