https://www.nbcnews.com/mach/science/zoo-hypothesis-may-explain-why-we-haven-t-seen-any-ncna988946?cid=sm_npd_nn_tw_ma

'Zoo hypothesis' may explain why we haven't seen any space aliens
The hypothesis holds that they can see us, but we can't see them.

March 31, 2019, 11:57 AM GMT+2
By Seth Shostak
Ask your friends why scientists have failed to find extraterrestrials, and you can be sure at least one of them will offer the following answer: Humans are not worthy.

We’re flawed beings. We routinely threaten one other, not to mention other species and the environment. That doesn’t sound very civilized, and it offers a plausible explanation for the lack of alien contact. Perhaps the extraterrestrials know we’re here but don’t want to deal with us ― either by communicating or by visiting.

This idea is endlessly appealing. It’s also old. In 1973, MIT radio astronomer John Ball published a paper in which he suggested that the lack of success in uncovering cosmic company wasn’t due to a lack of aliens. It was because these otherworldly sentients have agreed to a hands-off policy.

They’ve kept their distance not because we’re imperfect, but because of our right to pursue our own destiny. Diversity is something that everyone in the cosmos is assumed to value, so life-bearing worlds should be left to their own evolutionary development.
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