Because keeping the Earth sustainable means reaching consensus among 200 states and 8 billion people? That is a political/legal problem, and I am not sure why it should be "much, much easier" than first settlers terraforming an otherwise empty planet.
It seems wrong even to compare those two tasks. They are so different that they don't seem to have a common metric. An analogy: is is easier to stop two spouses from quarreling or to write a SHA-256 implementation from scratch? How would that even be measured?
At least writing a SHA-256 implementation from scratch is theoretically possible. The point is, it is possible to keep Earth habitable, were just not doing that. But there is no plan B. In other words, we're fucked.
It seems wrong even to compare those two tasks. They are so different that they don't seem to have a common metric. An analogy: is is easier to stop two spouses from quarreling or to write a SHA-256 implementation from scratch? How would that even be measured?