[mtVessel]

The post doesn't explain how the niches themselves come into being, so an equally logical assumption could be that creating a person also creates a new niche, and their task is simply to find the right one. No new person, no niche.

A less rigorous explanation, but probably more to the author's point, is that people who end up filling the wrong niche add more negativity to the world than those who find their place. This is a worse state of affairs than leaving some niches unfilled.