Because global warming of the kind we are causing/experiencing is quite literally unprecedented in human civilization. We have no method currently feasible at large-scale to undo the damage we've created.

It is going to be horrible, no matter what we do. It does have the potential to be civilization-ending. That's pretty much the end of humans as an advanced species.

If we lose our current level of civilization, we will likely never truly become a space-faring species and will instead end when the Earth does.