Goddamn it. I had no intent of sharing this article because its whole "in the beginning, there was darkness" approach to simple passion is really a turn-off for me. But this is a great discussion to have. Full disclosure: my 2-year-old daughter hit the "whys" early, at about 16 months. I answered every "why" with "why not?" and found my daughter toddling about going "squirrel! Why not!" and "Going swimming! Why not!" which was one of the most affirming things I've experienced in my life. That said, "why?" isn't necessarily a bad question. Elon Musk's "why" is similar to yours - he feels that humanity is doomed if we continue to keep all our "eggs in one basket" and that without the concerted effort of dedicated visionaries, we will be the eventual victims of a Tunguska or similar. The man has stated publicly how much he loves The Moon is a Harsh Mistress and Stranger in a Strange Land by Heinlein; the probability of his not having read "The Man Who Sold The Moon" is effectively nil. Elon Musk wants to give the stars to humanity, pure and simple. But it could be argued that terraforming Earth is a hell of a lot easier than terraforming Mars, and that a tiny effort here is worth a heroic effort there. Securing the safety of a planet that gives six billion of us a place to live should count for more than shuffling a million of them off to a place that makes Antarctica look like Eden. But you know what? It's not my money. And to be fair, Musk has put a great deal of effort into reducing our dependence on oil. His is not a singular approach. There are reasons to ask why, and reasons to ask why not. I find that the more simply you can ask them the more honestly you can find the answer. This whole "whoa! .gifs from Song of the South to explain orbital mechanics!" schtick is the most annoying approach I've ever seen.
That was my point though - I don't personally believe that humanity is done for if we don't colonize other worlds - nature is a tenacious damsel, and we're at least half as tenacious as it, we'll find a way. I'm saying that it's the "public, noble reason" he put forwards. Deep down, I'm not certain his reason isn't "I'm going to Mars and that's that". Sure, his actions will benefit all of humanity - but his plan, first and foremost, is to either have himself or another person step on Mars just 'cause that's what he wants to do. And that's the nifty thing about Earth - in theory, I'm pretty sure it's self-terraforming. From sunlight, thunder and elements, it created life. And then that life proceeded to survive 5 extinctions events. Humanity itself survived one - and without the tech we have today. There's only a few ways I know that could end that cycle of survival - and 90% of them is us, and we REALLY have to try (and life will be unbearable way before then - if humans have a certain driving force, it's absolutely comfort). As for Antartica looking like Eden - IIRC that's a stretch, as there's some places on Earth they use for simulating Mars - the only big disadvantage is that the atmosphere is thin and not breathable. Venus, on the other hand... Anyway - sure, "why" isn't a bad question - I just find it tends to railroad thought threads. And I completely agree with the "Whoa" thing.
Perhaps the bigger disadvantage is Mars's lack of a magnetic field to deflect cosmic rays, which will result in radiation poisoning for anyone attempting to reside on the surface. A Mars colony looks like a foxhole with an antenna. Not too sexy. To say that some terraforming is needed is an understatement.
That's nothing we CAN fix though - not without geoengineering, we most likely would be better off by generating that field ourselves somehow. But yeah, I DID forget about the cosmic rays.