Why do the scientists think that extraterrestrial life is possible if and only if the planet has all the conditions same as the earth has?
Most scientists look for theory's then back them up with evidence. Hard facts. In the case of proving life on other planets that is quite a tough question. Since the universe is so big and there is an infinite combination of ways that life could exist. Probably in ways that we couldn't even dream of. It is of course then easier to prove life exist by searching for other planets similar to ours. It cuts out all the guess work needed and lost time searching for life on planets that are completely different to ours, which might hold life, which of course would be completely different to us.
Our technology is also our biggest set back. We can't see planets in solar systems, in other galaxies on the other side of the universe. We can't even see planets in our own galaxy. So how can we detect life, if our tools are yet so primitive. If we listen, you have to remember that sound travels slower compared to light. Apparently in space sound doesn't travel well at all. Light goes fast, but by the time it reaches us, its already happened. We can't see what is happening right now on any planet anywhere out with our solar system because of the time it takes. So to find life, we have to go look for it. Physically go there, but of course the technology barrier again, if we try we would just get old and die before we got there.