Nerevar-as wrote...
Now you mention it, who is to say they didn´t? They are older than 37.000.000 years, I think Andromeda is 2.000.000 ly from the Milky Way, and Reaper FTL is rather fast if we go by ME2 final video.
It's a perfectly fair question, but that still doesn't account for all the other galaxies around. And I mean, really, if after 27 million years no perfect synthetic species from another galaxy has materialized to omnomnom on ours, you'd really think that the Reapers would go "Maybe we should overthink that whole reaping thing..."
And, please, nobody say "But they are only following their programming...". They are supposed to be true AI, remember? With own thought processes and all. You'd think that they spend their 50.000 years debating a bit among themselves, not only watching re-runs of the A-Team.