I would assume cos as humans with empathy for our children hard wired into us to the point that we will kill to protect them (insert some quote about natural selection here). As humans with empathy we can empathise with how that feeling is for others (a feeling a non parent can never understand). As social pack animals we are programmed to ensure the future of the tribe thus the collective protect the young even if the young dont nessasarilly have ur genes. Thus these feeling hardwired into us by evolution may manifest in the behaviour u have witnessed.

Ie. Without well adjusted and cared for children our society is fucked. And most people realise that.