It's much more economically viable, but it comes with a lot of ecological and environmental risk that we care about a lot more now (which is right and good). Same with preserving Antarctica, the other one people always bring up. The fact is we care hugely about the environment in the modern day, people who don't care about the environment are a loud minority, so in terms of making new habitable land on a large scale space is the most realistic even though it's more difficult in an economic and engineering sense.