I see this sort of statement all the time, and we have no empirical evidence it is true.
As this article says, even this simple "Starshot" concept is "a monumental engineering challenge that will require huge leaps in technology for spacecraft design, propulsion and communication."
And all the challenges are fundamentally physical, involving issues of weight, strength, density, power density, thermodynamics, etc. Unless they make some near-magical breakthrough in physics, a civilization one million years older than our own would find it just as difficult as we will!
And what if some galactic civilization has already done this? Are we likely to detect a spacecraft "just 3 to 12 feet across and weigh as little as a thimbleful of water" passing somewhere through our solar system at 20% C?
I don't think physics is a roadblock for space exploration at this point. We just need to figure out asteroid mining and building in space, after that we can send robots driven by AI to other systems, at low speeds. Once they reach other systems they would refuel, build more robots and carry on.
It's a massive engineering challange, but it's quite predictable within a few thousand years (if we don't destroy ourselves by then).
• That it is possible for a region as large as even a single galaxy to ever be "full" of one species or their creations.
• That anything can keep spreading indefinitely, and remain intact, not only across the gargantuan distances involved but also over potentially millions of years for other species to encounter them.
• That intelligent life wants to keep spreading or exploring indefinitely. As opposed to, say, creating VR universes of its own and attaining contentment within them.
• That there are only a few "directions" into which species or machines can spread across the universe, for them to run into each other.
• That there are no "stabilizing" forces – natural or adversarial – which counter rampant expansion.
The entire universe is limited to the periodic table. You would need to have a device made entirely out of material that has a half life greater than millions of years, and a battery that can last that long without sunlight. I'm not saying it's impossible because technology is often surprising, but that's a fundamental limitation that so far we know cannot be overcome.
Edit: Even if the robot behaved in a self-sustaining manner, this further assumes a perfect, unobstructed trajectory and asteroids full of essential materials are on the route to earth.
It also assumes that it’s safe to broadcast your presence to the whole galaxy that way. It assumes a lot of unproven ideas and then hand waves at a “sufficiently advanced” species that has only apparently enjoyed technological advancement rather than personal or social advancement. Maybe humans as we know them would spread unmanned probes across the galaxy for the sake of doing it, but I have doubts that humans capable of th feat would do it. I suspect we’d be wiser, or we’d be dead.