We had the ability to travel to the strs back in the late 50's/early 60's. It is recognised that the Nuclear Test Ban Treaty ended the Orion Project - LINK- as it used nuclear propulsion, and there were fears of fallout heading back to earth. Had it gone ahead, we might possibly be approaching Alpha Centauri soon. Who knows, the motto was "Saturn by 1970". I often wonder if other species went down this path and decided to try it. And indeed, why do we never, ever see or hear of Aliens at this level of development? Why are saucers only stealthy little sports models with cloaking devices? Why no loud ugly clunky polluting spaceships?
It might be nice to have some retro aliens for a change, but then again, aliens with lesser technology are going to be a whole deal harder to explain. So much easier when one can simply ascribe the unknown to a higher power.