Why should they be here? That seems a stretch to me. We don't know the motivations by those who could send them.
For the opposite position (they shouldn't be here) to make sense, there has to be either no other civilizations, or our probe-sending habits are so rare that no other civilizations do it, or we are the first.
Any of those other stances seem vastly more improbable, as they assume that our sample-of-one society just happens to be ludicrously different than all others.
But I do believe it is technically possible in principle to construct and build von Neuman probes. Once they work inside a solar system, then work on making them withstand, say, a few million years in orbit and see if they still work, once they survive a few million years "inactive" (or constantly repairing themselves or whatever it takes), send them off to the stars. But it would take a patience and scale of industry we humans literally can only vaguely dream of.
> ...scale of industry
They don't have to do that. The probe just has to make more "child probes" before it dies. You don't need a huge industry if they are self-replicating. You need only one working probe to start, the rest handles itself.
And it has to survive for millions of years until it reaches a new solar system without something to "feed" on. In many ways it seems easier to make a Neuman probe which can survive within a solar system than something which can travel to another.
This is a fascinating talk that goes into the details of such an endeavor.
What if the time it takes to travel between stars is longer than most civilizations exist? What if we aren't that interesting to alien species or aren't worth the time and energy to discover? We've never sent a functioning probe to any other star and probably won't in my lifetime. And life may not be at the nearest star, it may be thousands of light years away. We may never have the resources available to do it.
The probe doesn't require its parent civilization to continue existing to keep working. It self-replicates. You don't have to "pick" where self-replicating probes go. Each can self-replicate, and you can get nigh-infinite probes for "free." A von Neuman probe finds its own resources.
We have only had complicated writing for a few thousand years. People have wondered if a technical civilization existed on earth a hundred million years ago would there be any thing around now as evidence. If it takes a million years of travel in the freezing, radioactive vacuum of space to reach another life form, you don't know that humans will be around long enough as a civilization to figure out how to actually build the thing that you are describing. I hope so, but I don't think its been proven to be possible yet.
Von Nuemann probes are possible in this physical reality for a relatively cheap cost and small time frame.
It makes sense to put probes that can watch for threats anywhere you think that threats can be. Species that do this have a greater chance of survival than species that don't.