Did we learn nothing from the trilogy?
Advanced organic civilizations will inevitably create synthetics that will inevitably rebel against their creators and wipe them out. Then they drop their mobile platforms and retire to a community server to calculate pi for eternity. Another organic species rises, discovers the server, and unplugs it.
/sarcasm
Or... maybe some Leviathan escaped to Andromeda and are keeping other civilizations there in check.
Or... maybe the Remnants were keeping other civilizations in check.
That was the point I was making too. I agree. Something would have to keep other civilizations in check, because of the lore of mass effect. They could have made intelligent civilizations a rarity in the galaxy. They didn't. They made them SO common that a handful reach spacefaring level in every 50,000 year period, which means that dozens must overlap in time with each other if we are to assume that modern humanities' 200,000 year existence is an average length before a species becomes sufficiently advanced.
Given the sheer size and age of galaxies, a single species should have arisen to dominance long, long ago. Which means even in Andromeda, they would have to account for this NOT being the case, if they want to introduce a similar diverse society of species, as there should be hundreds of thousands by now if they aren't going extinct for some reason.
And this isn't just bullshit either. Modern cosmologists suspect exactly that, which is why the Fermi paradox raises a legit and compelling question and not just a curious thought with potentially no bearing on reality. It isn't like the Drake equation in that the only assumption it really makes is that a single species would arise to spacefaring capacity just once, out of hundreds of billions of stars in a galaxy over 10 billion years old. To me, that's hardly an assumption, and more like a certainty. The sheer scale and age make it so.
And this is also why SETI is now proposing the idea of actively searching for the telltale signatures of Dyson spheres, as a post-biological civilization seems like a logical outcome for an organic race.
So I liked Mass Effect's portrayal of these real life topics, and how they addressed these greater galactic scale questions about the nature of intelligent life in the cosmos. I just hope they continue this with ME4 and don't **** on their own lore whilst moving to another galaxy.