There are two possible reasons for this that I could see:
1) Children can't care for themselves, and as such the game doesn't permit them to live by themselves. By killing off adults to elders, it creates the possibility of a situation where a child or group of children do not have adults in the house.
2) Young Adults to Elders will often have careers, and children obviously don't. If it killed off adults that an active sim works with, those employees would have to be replaced by other townies. Losing friendships in the workplace can have a negative impact on the careers of active sims, making it harder for them to get promotions and reach the top of their field. If you take into consideration that a lot of players like to change active households, the chances increase that players will encounter poor work performance due to deceased co-workers.
It's also a bit of work to cultivate friendships outside the workplace, too. By killing off infants instead of adults, it decreases the chances that the sim that dies is the friend (or potential future wife/husband) of an active sim.
