We do NOT know that. We don’t know how difficult interstellar space is to cross, we don’t know if self-replicating probes are even possible, and we don’t know what it takes for a civilization to get to the point where it can start expanding through the universe.
We also don’t know how many civilizations there might be. Maybe there are a million of them, but it’s a one-in-ten-million feat to get to another star and all of them are sitting in their home systems wondering the same thing we are.
We know nothing. We are babes in the woods, trying to figure out things that exist in a sea of unknown unknowns. There may be a thousand reasons why civilizations have not spread through the universe that we have yet to discover.
It’s fine to speculate, and to try to figure out reasons why we don’t see anyone. But every attempt should be qualified with the disclaimer, “But we know so little about th how life forms and the limits of interstellar travel and all the rest that everything we think about aliens could be complete nonsense.”
We have a serious case of recency bias. We think what we’ve learned in the past 100 years is bringing us close to understanding what’s going on. Of course, they thought that 100 years ago, and 100 years before that.
I’m guessing that astronomers 100 years from now will look back at us as quaint and relatively ignorant, just as we look at astronomers 150 years ago, who weren’t even sure if there was anything outside of the Milky Way.
Well, it’s a math equation, but you are right that it gets misused by people. It’s best to think of the Drake equation as a schema or method for organizing our thoughts around how to tackle the problem of extraterrestrial life. The terms in it are arbitrary, and any one of them can be broken down into many, many different terms. It’s basically a top-level working model for facilitating discussion, and that’s about it.
We have been able to put some constraints on some of the terms. We know a lot more about how many planets are out there, for example. We’re learning more about stars which tell us which percentage might be able to have planets with life. And so on.
But if you take a term like, “Number of planets capable of harboring life:”, you can break that down into a million more questions that need to be answered. Do red dwarfs count? How about tidally locked planets? Do you need a Jupiter in your system to sweep debris away? What about planets in regions that have been swept by Gamma Ray Bursts or Supernovae? What other things don’t we know about expoplanets that may affect this?