Sometimes I wonder if the reason we've not found any life might be because we just don't recognise it as such. It might be too alien; maybe it's such a large system that we don't fully comprehend it, or perhaps it moves at a timescale that we just cannot grasp.
We created gods in our image to explain existence. Anthropocentric as we are, we assumed that humanity was somehow special, distinguished from all other life on Earth. Now we're doing the same with the very definition of life. Life looks a certain way on Earth, so obviously it needs to look the same everywhere.
It makes sense as an outset though, you can only look for what we know to look for.
We're looking for high power radio signals leaking into space. With a structure that we recognize. Basically analog screams into the void.
But we're generating fewer and fewer of those, because it's more efficient to spread data across frequencies at low power. As we compress data and encrypt it, the signal begins to look more and more like random noise.
There were a few decades where our civilization was screaming into the void, but those are coming to an end. Because efficiency beats screaming into the void.
We're probably average. If alien technology follows a similar trajectory to our own, there will only be a brief window where it will be sloppy enough for us to detect.
That's assuming they develop technology like us. If they live in really short bursts they might never get to that point, or maybe develop something completely different. On the other hand, they might have really long lifespans, perhaps tens of thousands of years, and perceive time in a completely different manner.
We're looking for (relatively) high powered radio signals. Regardless of how alien critters evolve, they're going to have the same constraints on their radio technology as we do.
It is possible they'd have some reason for pouring a tonne of energy into loud radio broadcasts, but if they are using radio for communications (as our SETI searches assume), then they will have lots of incentives to stop screaming into the void. At which point, our searches will not find them.
What about, rather than communicating with electromagnetic waves, they communicate with gravitational waves? Or something else? There's a lot of dark matter out there, what if it's alien telecoms?
Its more a statistical observation, life/knowledge could form complex patterns anywhere in the universe, but for sure exists in our cells. So we look for that similar to what's proven, otherwise we're just wandering blind
We humans are made out of most basic atoms of the universe, so it does make some sense to assume most life to be carbon based. There could be more advanced silicon based life forms, but it would be much more complex.
Of course it might be that more advanced life forms would consider us as same level as we consider ants. How many conversations have you had with ants lately.