Humans tend to define intelligence, life, and communication based on our own structure -carbon-based biology, electromagnetic signaling, language, symbolic thought, etc. This narrows the scope of our search.
We assume other civilizations want to communicate, would use similar media (radio, light, mathematics), and would send signals we could interpret. This ignores other potential modalities (quantum, neutrino, gravitational, exotic matter, etc.) or entirely non-signal-based forms of interaction.
We may not even recognize signs of intelligent activity if they don't resemble our expectations, ie entire civilizations could exist in forms of computation or energy we can’t perceive.
We assume ET intelligences are aligned with our timeframe or curiosity. Maybe they don’t care to communicate, see us as trivial, or operate on million-year attention spans.
It may reflect less the silence of the cosmos and more the limits of our understanding, especially the assumption that we're capable of detecting or interpreting intelligence beyond Earth. A epistemic humility, or rather our lack of it.
It’s not about being shortsighted, it’s about everyone being constrained by the same laws of physics. Our models, however imperfect, are still unreasonably good.
> Humans tend to define intelligence, life, and communication based on our own structure -carbon-based biology, electromagnetic signaling, language, symbolic thought, etc.
I would posit that none of these properties are coincidences, and are in fact likely to evolve convergently in most if not all circumstances hospitable to life. In particular I very much expect ET life to be carbon based; I don't believe there's a true viable alternative outside scifi (hint: silicon ain't it).
> entire civilizations could exist in forms of computation or energy we can’t perceive.
Could they? Really? There aren't that many gaps in the Standard Model. The aliens could be made of dark matter, I guess, and remain forever undetectable, but that's not to far off believing in invisible fairy kingdoms. And it still wouldn't explain why the baryonic sector is so devoid of detectable life. Ethereal undetectable aliens don't mean regular ones can't also exist.
> Maybe they don’t care to communicate, see us as trivial, or operate on million-year attention spans.
This one I'll grant (sort of: what's the evolutionary path toward such entities arising?), but it's still weird that we haven't seen any sign at all of them. These entities live on million-year timescales but have no visible effect on their surroundings? Why?
And more importantly, why is that the only thing that happens? Because if it isn't the only thing, then the question remains of why can't we see anything else?