The problem is that it is at least 10x easier to fix Earth than to terraform Mars, and all the problems we have on Earth are going to be there as well.
That might be the point. Few people are going to bother spending resources to fix a problem that doesn't exist. And by the time such a problem arises on Earth (inhospitable living conditions due to e.g. supervolcano), we might not have the resources to fix it. Whereas terraforming Mars creates the incentives to develop solutions while we have abundant resources, which means that we have a fix ready and waiting if something catastrophic happens on earth.