That's exactly the problem with people's outlook on the subject. Too many people, even scientists, assume that only an Earth-like planet can support life. Such thinking is highly arrogant, and hearkens back to the days when Earth was the center of the universe.
The major problem is... we're not interested in any other kind of life.
To whit:
What meaningful dialogue could we have with gas bags living in the soupy atmosphere of a gas giant twice the size of Jupiter in a solar system a hundred light years away? They might be able to communicate in radio frequencies, but then what? They have no metals to create machines, no materials to form permanent records with (except, maybe, the floating carcasses of other dead gasbags)... they have nothing to teach us.
Or maybe silicone based life... living in rock crystals, communicating by electrical impulses. They have no eyes, ears, or concept of the outside world... no technology, no astronomy. Nothing.
This kind of life would be of interest in the name of pure research, but how would they develop technology? A civilization?
It's arrogant to think of life in purely terrestrial terms, but we know that technology is possible
given these conditions. And it's technological, intelligent aliens who can communicate with us, and, hopefully, teach us a thing or two about space travel, the meaning of life, or perhaps a better way of making Coca Cola.
That's why we look so hard for Earth-like planets. Scientists admit that life can take many forms, but they're more interested in forms like us... with a head on one end, with eyes, ears and vocal apparatii, grasping appendages, and technology.
Of course, if we find intelligent vegetative forms which build radio telescopes out of molecular carbon in interstellar gas clouds, that'll be fine, but otherwise, we search for people like us.
