How come it appears when astronomers talk of life on other planets they seem to use Earth as the only template?

Just out of curiosity of inhabiting other planets possible spoiler alert after watching ”Martian” a week ago, it seems there is a theme going that for life to be possible on other worlds that planet has to be similar to Earth, in distance from its sun, water, if not an abundance of it, etc.

An international team of astronomers has discovered an exoplanet in the star Gliese 832’s “habitable zone” — the just-right range of distances that could allow liquid water to exist on a world’s surface. [space.com]

Eight new planets have been discovered in the ‘Goldilocks’ zone of their stars, orbiting at a distance where oceans and life could exist.
The discovery doubles the number of small planets less than twice the diameter of Earth which are believed to be in the habitable zone of their parent stars.
Among these eight, astronomers say there are two that are the most similar to Earth of any known exoplanets to date.

[http://www.dailymail.co.uk]

Who is to say that life, that if life existed on another planet they would not breathe c02, or methane, what about having a normal living temp range of 130–170 deg. F.? What makes astronomers believe life ”out there” has to adhere to the perimeters of here, being Earth?