I'd much rather be on a cruise ship than a lifeboat, but that doesn't mean cruise ships shouldn't have lifeboats.

No serious advocate of settling Mars, Venus or anywhere else in space seriously believes it will be easier than remaining on Earth, nor are they suggesting that it means we can abandon the Earth entirely, or care less about protecting its biosphere. They simply understand that, no matter much of a relative paradise the Earth is, so long as 100% of humanity exists there then we are placing all our species' eggs at the bottom of a single basket's gravity-well. It will do us absolutely no good if we solve climate change, war, poverty, disease, etc. only to get wiped out by the next comet or mega-asteroid that smashes into us. And, statistically, eventually one will.