Perhaps they create lots of children, let most of them die shortly after being born (perhaps by fighting each other), and then invest heavily in the handful that remain. Once food becomes abundant, some parents elect not to let most of their children die, leading to a population boom.

In fact, if you squint a little, humans already demonstrate this: men produce large numbers of sperm, which compete to reach the egg first. Perhaps that would have led to exactly this Malthusian disaster, if it weren't for the fact that women only have a single egg to be fertilized, and sperm can't grow to adulthood on their own.

Specifically, since parents don't invest in their offspring they don't tell them what they've learned. Thus no matter how smart individuals are, knowledge doesn't pass to the next generation.

Maybe the alien species has some other form of sharing information. For example the parents may share the knowledge with anyone, and later someone else will tell their children.