Why do so many scientists assume that the physics we know today is all the physics that there is?

Specifically, one of the tried and FALSE arguments scientists put out is that since WE today do not have the science to travel near the speed of light, that therefore it is not possible for any intelligent species out there to have this science either and that therefore it would be impossible for them to be able to reach Earth. Why are some scientists so closed minded? Did we not have intercontinental flight a century ago?