[v3rt]

I believe the predominant evolutionary theory actually does have an explanation - sexual reproduction serves to produce offspring that are different from their parents and have a better shot of being adapted to a constantly changing environment on average (particularly relevant to parasites/diseases). These children still carry their parents' genes, though, so it is in the parents' best genetic interests to die once the kids are self-sufficient so as to avoid out-competing their own offspring.

[asdflkj]

W.R.T. humans, the predominant evolutionary theory does have an explanation, and it's exactly what jwegan said. Your scenario is only relevant to organisms who live in niches very far from our own. Humans have always lived in ecosystems large enough that our kids' share of the burden of our claims on resources is infinitesimal, and certainly no comparison to benefit that kids derive from their alive parents.