The biological plausible explanation is that there simply has not been enough time for other civilizations to form up. It took us 4 billion years to reach civilization from primordial goo. Apparently not many planets remain habitable that long. It does not matter if you have a trillion planets if they don't remain stable long enough.
it's not even entirely a given that we are the first, as it's unlikely any evidence of a small tribe of sentient dino villagers for example would persist to the present day.
Suggested reading: Rare Earth: Why Complex Life is Uncommon in the Universe by Peter D. Ward and Donald E. Brownlee. The basic premise is that complex life is incredibly rare, because only through a series of effectively chance events, did complex life happen on Earth.
Even if each of those events were actually inevitable, planet size, radiation received from host star, exact chemical makeup of atmosphere/oceans and any number of other variables could affect the time scale for a planet to fruit.
But on the universes timescale, a slight variance could still be thousands of years or more. So maybe there is life in some distant reaches of the universe, who are +/- a thousand years from our level of development. Their signals simply haven't reached us yet, or ours them.
And the odds that it’s us are astronomically even longer than the odds that I’m going to win the next Powerball, and I’m probably not even buying a ticket.
It is also stupendously implausible that our civilization arrives at all in a mere 14 billion years, considered against the gazillions of years in the potentially infinite lifespan of the Universe. Yet here we are.
If it is difficult for civilizations to develop, then our existence is so implausible already that being the first is not such a leap.
I did not imply at any point that we were the first, or only. Perhaps in our galaxy. I don't know if we could ever find out about civilizations in other galaxies.
There are a 100 billion galaxies. I'm ready to guess each has at least a few planets theoretically capable to sustain life for long enough.
