Small children are well known to be afraid of voids (closets, under the bed) in their sleeping area. Knowing this, why don't we design children's rooms to eliminate them?

I'm not a parent, but going by pop culture, it seems like literally every child has the same fears.

In pre-modern times, I imagine that they'd be sleeping in the same room as the parents, but if modern notions of privacy don't permit that, seems we could at least design an enclosed capsule or something.