[ezrast]

If we were completely asocial, independent creatures, then perhaps. But humans - and therefore their genes - have always lived and died in groups, and even a primitive tribe may stand to benefit from having a small amount of individuals unburdened by child-rearing. From the point of view of the gene, what's important is only that some part of the family tree propagates; the number of dead-end branches doesn't matter. Therefore an increase in reproductive drive could easily be deleterious to a particular gene's long-term survival.

Disclaimer: I have no knowledge of the role of child-free individuals in our era of evolutionary adaptedness. I'm just pointing out that evolutionary dynamics are a lot more nuanced than "more children = good", especially in social animals.