Fine, Jack, but it's still only a guess. The fact is, we don't know for sure if there are any habitable planets around the nearest stars. Correct?

Has anyone considered that other civilizations may have found a form of space propulsion which does not burn any fuel?

Burning fuel is what we are accustomed to now, but it's very probably not the only way of propelling a vessel at high speeds, and there may be a fuel-less method of travelling at speeds in excess of light speed. Just because we think it's impossible does not necessarily mean it is.