Let's start with the biggest: Mars settling is not. about. throwing. away. Earth. Heck, it's not even primarily about a backup for Earth. It's about starting a new civilization to expand the human experience and make the future a more interesting, expansive place. Literally 99.99999999999999999999999998% of the universe is not Earth. Mars would just be our first real step out, the first place we establish life outside of Earth (the point of terraforming, after all, is to cultivate Mars into a garden, a second full biosphere). It's the same kind of argument for why people choose to have children. The idea of it being necessary for the continuation of the species is not the proximal reason for it. It's an adventure.

Second: Antarctica has a few small colonies that have been grandfathered in, but it is actually illegal to build permanent structures on Antarctica. In spite of this, Antarctica sees tens of thousands of visitors every year. The reason people aren't trying to settle Antarctica is that it's effectively illegal to do so.

Third: Mars has a significant atmosphere that at low altitudes shields against all micrometeorites and all the acute biological effects of even the worst solar flares. It can be easily captured to produce both fuel and oxygen and even some water (yup, you can pull fresh water right from the Martian air).

Mars is much better for settlement than the bottom of the sea in essentially every way except transit time.

Fundamentally, should humanity stay confined to a single planet forever or should we venture out? If the former, that's an incredibly dark future and we should thus cancel the entire human space program.

But I think we have a duty to Gaia to spread life throughout the universe while we can.