Can any of you who insist on thinking only in terms of alien _civilizations_ explain _why_ you do this? (ie all of you.)
Do you really think future developments in AI (science of mind and consciousness in general) and genetic engineering (both specific to humans, and in general as a branch of computing science and bio-machines) are not going to have any significant effects?
What do you think such effects might be, on the human species and our cultural structures?
Have you considered that such effects will be a universal constant, for all species that develop technology, no matter what kind of bodily form and cultural/economic structure they evolved. Fundamentally they have to have minds of some kind, and some kind of chemical genetic coding. The details don't matter. Once they become able to manipulate and extend both, the same technological singularity occurs for them. When sentient individuals of any kind attain the ability to self-bootstrap to something greater, at least some of them will attempt it, and some will succeed. Regardless of any social restrictions or opposition. Sometimes peacefully and openly, sometimes in secret to avoid conflict, sometimes in outright conflict with the rest of the culture.
Knowledge cannot be effectively repressed, while society retains any technological continuity. So the scenario repeats over and over, until one of the conflict instances results in termination of the potential for further instances of individual self-boostrapping.
Falling back to pre-industrial state doesn't count as 'termination of potential', since it won't take long to regain the technologies. Termination of potential means destruction of the species, maybe even the world environment. Given that one or both sides of such conflicts have bio-engineering, AI and nuclear tech, that's a certain outcome for some such conflict as they repeat over and over.
And there's the answer to the Fermi Paradox.
Do you _really_ think the Star Trek like scenarios are anywhere near feasible? So they have just a few kinds of tech advanced over ours (warp speed, photon torpedoes, teleporting, etc) but nothing which makes a 'society of plain old humans' untenable?
Even the Borg are an infantile concept, totally failing to integrate absolutely predictable technological developments.
Btw, speaking of inscrutable aliens, I recommend the short SF story Roadside Picnic, by the Strugatsky brothers.
http://soviethistory.msu.edu/wp-content/uploads/2015/10/picnic.pdfThe 1979 Tarkovsky movie Stalker was based on this story, and is very very good. Also, making it killed most of the film crew including Tarkovsky. Some of the scenes were filmed in the area of an old abandoned power station. PCBs and other contaminants were in the water, and most of those who stayed there died of cancer over the next few years. So it was Tarkovsky's last masterpiece. A tragedy.