There seem to be no great obstacles to intelligent life devouring the galaxy. So why are we here.
If life on earth arose on earth , and produced humans in a few billion years, why not on some other planet ten billion years ago?
Simplest and most likely explanation is that life is unlikely – requiring a stupendously improbable assemblage of molecules to form.
No one has constructed a plausible high probability origin of life.
Indeed, it looks to me that water based life just cannot form spontaneously. The minimum complexity of life in water is just far too high. RNA based life just does not work without DNA and cell membranes, and you don’t get RNA and DNA spontaneously forming in water.
Here is what I think happened:
There is some environment, perhaps a mixture of liquid cyanide, liquid formamide, and polyphosphoric acid with star tar dissolved in it, in which life can form spontaneously.
Cold temperature origins seem most likely, since cold temperature life can easily spread from planet to planet, because cold temperature planets with liquids are smaller and lower gravity than warm planets with liquids. Volcanic eruptions etc can easily spit rocks into space.
This low temperature, non water based, life evolved, over ten billion years or so, to adapt to environments increasingly alien to its origins, eventually becoming water based life living in hot deep rocks on asteroids.
From which it infected earth. To produce complex life, you need an oxygen environment so that cells will gang together for defense and attack. For an oxygen environment, you need a water environment. We are the first, because it just took that long. And, in due course we, or some other earth species if we fail, will devour the galaxy.