It only takes one small group within one species to generate a self-replicating probe, at which point all of the galaxy is visited in a few million years.
A solution to the Fermi Paradox has to explain why that event has never happened despite the fact that some element of our species will do exactly this at some point in the next thousand years. The laws of physics allow it, and our psychology is clearly up for it.
The largest radio telescope we have (the 305 meter diameter Arecibo) would need to have it's sensitivity increased by around two orders of magnitude JUST to pick up our TV/FM/AM signals from outside the solar system. If we move into the narrowband signals then, depending on the source-strength, it could pick up signals at up to a few thousand light years... if it happened to be pointed in exactly the right direction at exactly the right time. So, our most sensitive instrument is only capable of measuring a fraction of a percent of a fraction of a percent of the galaxy.
I just straight-up don't believe that we're even remotely approaching the capability of asserting that the galaxy is sterile of higher life that's constantly dumping EM noise... and that's without assuming they've found ways to transmit data that we're ignorant of or that we've seen it and just haven't noticed it.
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The other thing (and I feel a bit like a religious person saying this) is that we have exactly zero basis for asserting that the type of machine you're talking about haven't been here. Given that if one of these machines came here, and stayed, the Earth itself would annihilate all traces of it (especially on the surface) within short order we shouldn't necessarily expect to find evidence of a visit.
Taken to the extreme, you could even reason that all life on Earth could be the product of one of these machines.
Hah, hilariously ironic. "Where is alien life in the universe? Are we the only ones?" continually bemoan the descendants of the organic probe sent to Earth to populate it with intelligent life.
And what would that probe look like?
For it to be a useful to the sending civilization, it needs to go out, replicate, and send information back.
The first two are easy. Send out lots of meteorites with bacteria on them.
The third is not so easy. Creating a self-replicating machine which can then transmit back across interstellar distances or store information in its replication program and then send more of itself out is hard.
Or, you help life evolve and then wait for the results to reach the point where they can send out their own probes and signals...
And more importantly why would anyone send anything to anywhere?
I never understand the argument people make about probes spreading across galaxies. If we assume they are at least as intelligent as we are, then purposelessly wandering in the universe would be least of their goals.
I can understand if probes have military goals. Like for example to evaluate threats to them. But like us here on earth, and with their technology would be carried out at a technical sophistication we have no remote capabilities of detecting currently.
You're assuming everyone is rational. For instance, if I were a future billionaire I'd totally do it... knowing I'd never see even a glimmer of return on the investment... just cause.
How do you know that it has not? I'm not seriously asserting that it has, mind you. Just noting that there could be a swarm of radio-silent probes in the Kuiper Belt, in which case we would have no clue whatsoever.
One'd expect self replicant things to grow into every available ninche, however irrelevant it is. Thus there's no self replicant thing consuming our galaxy (or there is, but it's incredibly new).
Another idea is that basic game theory and physics applies to everyone. It's virtually impossible to defend against attack by objects accelerated near the speed of light. The only way to be safe is to destroy potential threats before they destroy you, and be very very quiet.
