It isn't. Earth has those temps. I guess you're saying that Earth isn't habitable?
-100 is totally within our abilities to make a habitable sanctuary and, from there, were can work on other adaptations while we try to terraform the atmosphere over generations to increase surface temps.
An exo-planet with temps far more extreme than that could be described as "the most Earth-like ever discovered!" until we knew more. If you think otherwise, you're kidding yourself. When they describe a planet in the habitable zone, they are saying that it COULD have temps similar to Earth. There are a lot of factors that they wouldn't know, which is why it's a whole zone and not a more specific orbit for the star's level of energy output. Is it geologically active? Does the atmosphere have an appropriate greenhiuse effect? Where exactly is it in the zone?
Mars is always losing atmosphere. It was far different when it had oceans (assuming it did). Back then, it was more Earth-like than Earth, so we need to find an Earth-like planet that we can reach during or before it's period of being habitable. At this point, Mars isn't so far gone that it can't be terraformed for long enough for us to adapt to some of the more permanent changes.
We might engineer people with lower body temperatures, lower metabolosm, able to breath with less oxygen, and more tolerance for cold (penguin genes, anyone?

requiring very little water specifically to deal with a terraformed Mars.
Terra and genetic-forming.