Because there is a bias that cannot be shaken. Look its easy to not give a shit about other peoples kids. Let other people's dirty nasty misbehaved little shits get nothing free in this world. When it's their own children though, people begin to apply exceptions. And that's why we end up with hereditary based transfers of wealth, rather than say donating all your money to all of society upon death. Your own child that you raised from birth deserves your wealth more than the wretched society (basically other people's kids).

If someone else's child was run over and killed in a road, a person may pause to reflect on how sad and unfortunate the event was, but will quickly move on with their life and continue as they were. If their own child though was run over and killed though, they would be shattered, may not even be able to function well for quite some time.