Is it better to starve for scraps in a dead world of dwindling resources or to strive to find a new one?
Realistically irl, it’s way easier to teraform a mostly fine planet with a working magneto sphere and liquid water as opposed to going to another planet where you’d have to do all that yourself, but sci-fi likes to pretend there are thousands of lush green worlds out in the endless black waiting to be colonized.
The larger issue would be the lack of easily available fuel to rebuild earth with, an issue only made infinitely harder by being light-years away from home and starting on a new dead rock instead of a mostly dead one.