Humans are by far the most intelligent creatures on this world, and thanks to that intelligence can learn a lot.
Learning primarily takes place in the childhood stage, when parents directly teach their children all they need to know to survive (which until not so long ago, was indeed mostly survival skills: how to grow your own food and so). A long childhood (and with that, long parental care) may for this reason be an advantage: longer time to learn typically makes for a better end result.
As another poster pointed out, childhood is not the most robust stage of the life cycle of a human, especially early childhood. And even if parental care during childhood improves survival during that stage of life, it's genetically not exactly a productive stage of life - no procreation yet, so surviving that part is great but it doesn't necessarily help to spread those presumably beneficial long childhood genes in the overall population. Procreation tends to happen when the parental care has finished.