They're made out meat.
Not necessarily meat that agrees with our biology.
It's a short story that tries to explain Fermi's Paradox, IE "If Drakes Equation shows that there are tonnes of alien civilizations out there, why haven't heard from them".
The answer, according to the story, is that WE are made out meat. Who wants to talk to meat? That's gross.
We have only had the capability to listen for ~100 years. We have only been actively listening for ~50 years and concurrently stood on the brink of wiping ourselves out.
Now, another civilization has to have been broadcasting at sufficient power in our direction for us to hear them over the noise floor and have been doing so at a time enabling us to hear them while we were listening. If another civilization developed radio while Rome was busy transforming from a Republic to Empire and wiped itself out* by the time Rome fell to the Goths, there is a ~500 year time span for us to hear them. If they are ~1500 ly away, we have may have missed them. If they are 2000 ly away, we should just now start to hear them.
I think it is fair to say that there are likely no radio emitting civilizations within ~50 light years of Earth, but we also do not scan the whole sphere of the sky in across the spectrum continuously. So we aren't even listening all that well.
*There is an option for the civilization to abandon the EM spectrum for some other means of communication very rapidly and appearing to have been wiped out.
The problem is not real-state in space, nor the quantity of resources in said real state, but the "habitability" degree of it....
Unless we start developing real ecopioesis or a better way to apply RIS (resources in situ), we are stuck to other "earth-like" planets, which i bet are much less than the most optimistic value in the equation.
The term used in "Case for Mars" and at NASA is In Situ Resource Utilization (ISRU) and there has been a lot of effort put into ISRU of both Lunar and Martian resources.
There are those that argue we shouldn't actually crawl back into the deep gravity wells of large planetiods at all, carving out Asteroids and building O'Neill Cylinders (Lagrange point colonies).
Artificial worlds will be built and their inefficiencies will require trade. Basic commodities like water, nitrogen, sodium, etc will become important raw materials. These artificial worlds will either mine those commodities out of uninhabitable worlds or produce finished products from raw imports.
(I know a lot about this stuff...)