This is pretty cool. I am constantly looking at current events and thinking to myself "what if aliens were watching this?". I feel like it sorta puts things in perspective for me. Seeing that this might be a possibility is sorta thrilling.
There is an archetype of the Mysterious Stranger, an innocent, naive observer of our culture, who finds us incomprehensible because we are Just Terrible. We make war, pollute, enslave, etc, while this archetype is pure.
Examples: K-Pax, Powder, Dostoyevski's The Idiot, Crocodile Dundee all have elements of this archetype
The thing is, the only reason this archetype has any meaning to us is because this archetype shares our values. That character is always just the author, putting words in the mouth of a character who could not exist in reality. There is no Mysterious Stranger.
We can expect actual aliens, ones who evolved in a different ecosystem, would have no opinion about what we do. Not from our value system anyway. Less "these poor hooman creatures actually kill each other, how primitive!" and more ... well, something more incomprehensible to us
TVTropes has a whole section called "Blue-and-Orange Morality" for this: https://tvtropes.org/pmwiki/pmwiki.php/Main/BlueAndOrangeMor...
I mean, it's possible aliens are so similar to us we could communicate and share values. I just don't see why we would expect it, given that they will have evolved in a completely different environment with completely different evolutionary pressures. We would be far more likely to figure out what whale songs mean than communicate meaningfully with aliens
I'll leave aside my personal favorite pet answer to the Fermi Paradox that we have not evolved the ability to even perceive aliens; but suffice to say when I mean aliens are different, I mean different, not just basically people with funny ears
So, to my mind, the question to answer is not "Why wouldn't aliens?" but "Why would they? How would it even be reasonably possible?"
Now my favored theory is that there are no other sentient species within a detectable range from us, and same goes for any of them out there too. Time to start accepting that we’re (functionally) all alone out here.
I'll define sentience for us quite loosely and vaguely to mean "has self awareness" and "knows stuff that would be useful to us", like new fields in mathematics and physics. LMK if that's incorrect.
There may be universal aspects to that, and we're only speculating, so you could be right ¯\_(ツ)_/¯ I have a feeling that "sentience" as a general concept, whatever the definition, contains these "universal aspects" right in the definition, begging the question.
We only have a single data point for even recognizing what sentience is: as you say, some indicators are "motion", "wanting stuff", "energy usage"
Since there are example on Earth of creatures that have all of those traits that are definitely not sentient, can I assume you mean at greater scale? So, motion across interstellar distances and what that implies.
To my mind, motion across interstellar distances is neither necessary nor sufficient to demonstrate "sentience" as defined above. I can easily imagine a mindless entity evolved to spew spores into space with only enough sentience to take advantage of gravity sling shots to speed themselves along the way, that we mistake for spaceships. Or highly advanced, sentient creatures that are immobile, that look to us like barnacles or slime mold, that take thousands of years to complete a single thought.
At the end of the day, I believe we are going to recognize only creatures that evolved under very similar circumstances to us as "sentient": can manipulate objects; can emit signals we recognize as such, at a scale we can comprehend; bonus points for spaceships, clothing, machinery; bonus points for using scientific principles that we do not yet know, but would find useful.
I suspect (admittedly without due cause) that this kind of sentience is exceedingly rare relative to the potential diversity of what is possible; which is to say, that post-first-contact humanity will find our definition of sentience to be astonishingly narrow
... besides ourselves. It's not that we don't relate to anyone or anything this way; its that the relation is reflexive.
Deepest visible light image of 10,000 galaxies - https://www.spacetelescope.org/images/heic0406a/zoomable/
NGC 1866 star cluster - https://www.spacetelescope.org/images/potw1847a/zoomable/
Runaway galaxy UGC 10214 with thousands of galaxies in backdrop - https://www.spacetelescope.org/images/heic0206a/zoomable/
Abell 2218, a rich glaxy cluster composed of thousands of galaxies - https://www.spacetelescope.org/images/heic0814a/zoomable/
Gordo ("The Fat One") - enormous galaxy cluster with mass of three million billion Suns - https://www.spacetelescope.org/images/potw1802a/zoomable/
Gallery of more of these at https://www.spacetelescope.org/images/archive/zoomable/ - follow a delightful thumbnail, then desperately seek the tiny buried zoomable links on subsequent pages.
This seems like a false dichotomy. There could be 200 billion planets with life in the universe...but only about one per galaxy. Someone did a probabilistic analysis of the Drake equation, and found out that indeed, given the known facts (that we don't see any life yet, how life appears to have developed on earth) that a substantial likelihood exists that the density of life is right in the range where there are many instances but we will never see one. It's sad to never get closure, but it's irrational to deny it's a possibility.