the most meager terraforming takes hundreds if not thousands of years using science that we don't even have yet. How does one even contemplate a public works project that would last longer than any government has ever lasted.
If you could sustain such a project, how could you justify the cost over a period so long that the finished product might be obsolete long before completion.
Our future in space will be on interstellar earth twins or near twins, assuming there are any and we figure out how to get there. In this system it will be man made habitats, assuming we find the treasure out there to make it worthwhile.
We will never leave earth for the sole purpose of insuring the future of humanity but cheap access will eventually get us out there for the money.