One thing I don't fully understand is why single family zoning is necessary to make this a reality? Won't density naturally fall off as you move out of the core

One thing I don't fully understand is why single family zoning is necessary to make this a reality? Won't density naturally fall off as you move out of the core? Since area increases with the square of the radius, it seems to me that a larger core means more core will be accessible to more suburbanites commuting in from surrounding areas. Couple that with an architecture that can better support things like commuter rail, and it just feels like a situation where everyone wins.