Terraforming

Latji was one of those worlds where terraforming hadn't quite caught, so even at the equator the few inhabitants huddled in tunnels, away from the arctic cold and snow. The market reflected that, being mostly deserted and boring despite sitting right next to an interstellar traffic hub.

...

They went on through the uniform white landscape, broken only by rock formations and smoke stacks from which black clouds billowed up into the sky. In another century, there could be enough CO2 in the air to trap a little heat for a change... and no ozone layer. Oh well, thought Jake, you can't have them all.

— Arrow in the Sky

There wouldn't be much point in settling distant planets if they couldn't be made friendly to life. A sealed habitat can be built anywhere, much closer to home and more cheaply. But it will always have drastically limited capacity, and permanent morale issues, to say nothing of the fact that a perfectly closed ecosystem is in fact a chimera.
So why not settle a planet that already has life on it? Because it would most likely be deadly to us... or else we'd be deadly to it. We'd need a fantastic coincidence to find alien life even remotely able to occupy the same ecosystem with anything coming from Earth.
Terraforming was therefore perfected as a necessity, to the point where it can start showing results in as little as a century or two. Of course, the entire process takes much longer, and sometimes it's never completed, either for lack of resources or simply because the planet in question isn't amenable enough. But even a partially habitable planet is better than a radioactive piece of rock.