Population and wealth are the reasons.
I currently have Luna, Mars, Mercury, Callisto, Io and Ganymede terraformed. Because of population limits on Earth, I farmed out half of my research to Mercury, all ordnance and fighters plus some construction to Luna, most of my fuel is Mars and Callisto and Ganymede also has a large construction base. Earth has all of my shipyards, half my research and most of my construction capacity.
I'm up to almost 4 billion in Sol now (started at 500m). I wouldn't be there and would have severe manpower issues if I hadn't done all of this colonizing.