User blog comment:Suparockr/List of Plotholes throughout the series/@comment-24163193-20141103163437/@comment-9753034-20141105114243

Interstellar travel may be possible with mass effect technology, but traveling between galaxies is a different matter entirely. Even if the Reapers could keep up their estimated max travel speed indefinitely, it would take them several centuries to reach our nearest spiral galaxies.

More importantly, though, we generally know very little about their history and what they've been up to during the time of their existence -- hence we do not know that the Reapers have not already done this. What we do know is that for a very long time, they have been the Catalyst's primary solution for overseeing and preserving organic life in this galaxy. It's perfectly possible that they've already scouted a large area around the Milky Way; whatever they found or didn't find out there, we wouldn't know.