"during human existence" is not an accurate measure at all.
earth is young. that doesn't mean the rest of the planets in the universe are.
the idea of space travel should also not be limited to the idea of limited means. it's likely that with an understanding of physics millions of times greater than our own (relative to our difference in age as a species), these civilizations don't have to deal with overcoming the problems we humans face as the pilots of jet-propelled craft.
then you need to consider the size of a UNIVERSE SOCIETY. i don't know exactly how long humans have been around. but i wouldn't call the notion impossible that a species, or very large group of different species co-mingling, could be millions of times older than humans. thus creating a possible population of numbers and vastness unimaginable. perhaps there are more of them than there are stars in the sky... perhaps enabling them, if they did not create all the planets in the universe themselves, that is, to keep tabs on all life-bearing planets.
and who knows, perhaps we're under an intergalactic quarantine, effectively disabling our evolutionary path... cuz we do SO MUCH STUPID SH*T.
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