Because there's no other intelligent life out there.
You know that. But it bears mention, for others following along, that rampant artificial intelligence or similar does not count as a great filter candidate, because it would be just as visible as the life it's replacing.
That doesn't leave a lot of fast-acting filters that could still be in our future, to be honest. Nuclear war? Sure, but we've avoided it too long for that to be a good one either.
Artificial intelligence would necessarily have different physical needs than biological life. We can expect even our own AI to be a bit alien to our ways of thinking. Alien created AI would be completely alien to us. With different needs and different motivations, what evidence do we even look for, and if it is pervasive, how do we know we aren’t looking right at it?
It would still exist in the same universe subject to the same laws of thermodynamics, and would still extract energy from fusion stars in a way that leaves an unmistakable technology signature.
It would be subject to the same laws, barring novel physics (like extracting energy from dark matter or some other process we don't yet understand). But a single AI could be smarter than 10 billion humans and consume less energy, need less space, and not care about expanding out into the stars. They could coast around their host star indefinitely and when they eventually needed to leave (which could be never), they could store energy and make a long, slow trek to another star. Imagine trying to find an asteroid-sized object in interstellar space.
You know that. But it bears mention, for others following along, that rampant artificial intelligence or similar does not count as a great filter candidate, because it would be just as visible as the life it's replacing.
That doesn't leave a lot of fast-acting filters that could still be in our future, to be honest. Nuclear war? Sure, but we've avoided it too long for that to be a good one either.