lots of obstacles to become human level intellectual
- have life
- have multicellural (or similar) life
- do not get hit by a huge asteroid all the time to wipe you out
- become intelligent
And million other smaller obstacles.
So yea that could be it. We could be the only ones in the closest 15b ly of the universe.
Self replicating probes could colonize the whole galaxy in relatively short time, yet we see no evidence of them. We see no alien radio waves (we've almost stopped emitting them ourselves, data moves as light in optical cables). We see no dyson spheres. Nothing.
Maybe there is a lot of life out there, but it never gets as complex as it is here. We are the product of so many chances that it is amazing. Anthropic principle at work.
Or maybe advanced aliens have no reason to make themselves noticeable or roam the galaxy. We just don't see them. Or they are so different from ourselves, we don't notice them.
It can be any of the above tbh.