Because I sincerely doubt that terraforming a new planet into being habitable and transporting 8 billion people there is easier than fixing the perfectly good planet we already live on. If habitating a new planet was more feasible, then sentimental reasons would be it, yeah.
because if we can't make it work here, under the most ideal of conditions, we don't deserve it elsewhere. At that point we become the aliens from Independence Day.
As long as humans survive, why save the planet apart from sentimental reasons?