Why don't kids share their food?
Instinct. Their lives depend on food. They have no way of getting more for themselves. Young (of any species) who are wired to not share food would be more likely to survive.
Children who internalize a sense of the abundance of food will often voluntarily give away food to another. But they need to feel like they can get as much as they need when they need it. Their feelings might not match reality! From an adult view, there may be plenty of food yet they’ll still feel like they shouldn’t share. Or there may be limits but the child doesn’t feel them.
In the big picture, it’s evolutionarily practical for children to be wired not to share food.