Between Tesla (now including Solar City), his work on Hyperloop, Neuralink, and now The Boring Company he has plenty of projects focused on improving things on Earth. His goal with SpaceX is to make humans an interplanetary species. As long as we are entirely dependent on Earth, the future of the species remains at risk of being wiped out in a major extinction event.

The goal of putting a million people on Mars is hardly a mass migration when compared to the population on Earth though, if anything it is a small step towards establishing a presence somewhere outside of our home planet. On top of all that, it's very likely in the process of making Mars habitable we'll develop new technology to help repair Earth and improve our ability to live sustainably here. SpaceX may not be who accomplishes this in the end (it'll likely be a group effort), but I'm ecstatic that they are working towards it.