You're making a lot of assumptions about life, here.One, there could very well be Galaxy-spanning civilizations. But they may not be "life" as we recognize it. They may not even be carbon-based.
'Life Jim but not as we know it', yes that's great material for sci fi, but imagining this is actually probably real- is not assuming anything?! The universe all came from the same place and hence is all made of the same stuff, our own planet and solar system represents most possible conditions and habitats, in an ideal setting, and complex life can only just survive in a very very very narrow margin within this.
Back in the days of Verne and Poe, we wondered what sort of people lived on the moon, we'd be amazed at a fossilized microbe on Mars today, precisely because science has revealed how very special Earth and humanity is
Two, there could have been countless civilizations in various portions of the galaxy...and they were wiped out by *insert means here*. We'll only ever know about those civilizations through their ruins, if we find them.
Three, there is far more than just our galaxy out there.
Yes, but like microbes colonizing the ocean, the more planets colonized, the more resistant to being wiped out entirely. Perhaps alien colonists never quite reached the tipping point, beyond which they became immune to extinction.. That's always possible, but now we are walking a pretty fine line- as we often hear; the universe is teaming with just enough intelligent life, to make humanity insignificant in a very intellectually gratifying self deprecation- but just not quite abundant enough to have ever actually taken off to the extent of a logically inevitable colonization
again we have barely started to come to grips with leaving the surface of Earth, over little more than a single lifetime, and already we have craft leaving the solar system- in BILLIONS of years.. no one single civilization ever took this to it's logical conclusion? why not?
On the number of galaxies, yes there are lots, but ours is an exceptionally large, organized, hospitable, and dare I say, beautiful one- if we are alone here this does not bode well for life in the vast majority of scrappy tumultuous galaxies. Just as being the only intelligent life form on Earth, does not give Pluto great odds for changing that distinction