Wait, which UFO thing are you referring to? Carter never thought it was anything extraterrestrial, he stated that he assumed it was a natural or man-made event. Are you saying it’s strange that some people believe in aliens?
- velocity --- it would need to be moving quickly and would have to decelerate (requires energy) to do more than pass through
- life support/technology requirements and attendant heat signature --- interstellar space is _cold_ and not conducive to either, so requires on-going energy output
- size requirements --- basically, an entire ecological system needs to be moved around --- how many trees does one person require to produce sufficient oxygen for them to breathe? (a quick search has an answer from 1--8)
&c. See Kim Stanley Robinson's recent novel _Aurora_ for a well-researched examination of this.
With our technology, we could easily avoid being seen by a hunter-gatherer society.
Do you imagine that your society does not have similar blind spots?
How much arrogance there is in the modern Euro-centric world. Because the society has explored beyond what our grandparents believed possible, suddenly we are the pinnacle of the Universe.
Consider for example the "one-electron" hypothesis --- a far simpler take on it is that the electron is a fundamental particle and that we are nearing an end-game of understanding sub-atomic physics and realizing Einstein's dream of a grand-unified theory --- the universe doesn't seem to be shaped and formed so as to allow for FTL, so one instead needs to work within the bounds of converting mass to energy.