What you need to remember is that Taris is an ecumenopolis - an entire planet covered in cityscape. Very few full planets, even in Star Wars, boast the same - Coruscant, Corellia, Alsakan and Anaxes are some of the few. Even Nar Shaddaa is merely a moon, far smaller than Taris.
The resources required to recolonize and reconstitute Taris are a microfraction of the resources required to create an entire ecumenopolis even on an already mostly urbanized planet. The original infrastructure is, according to several NPCs, "Surprisingly intact" as well.
Actually incorrect about Corellia. It is not a city planet. It simply has some GIGANTIC cities. But much of the planet is open and even has mountains.