They make one Reaper per conquered race right? As others have mentioned they would need to fully invade a new galaxy not run into a race that could whip their ass or is already fully synthetic. They would also not have to run into a civilization that somehow overcame the synthetic-organic struggle peacefully which would completely overwrite their programming since the creation of the Crucible gave the Catalyst "new options", they would for efficiency sake have to not only clear out every single race in that galaxy capable of spaceflight (without the benefit of the Citadel/Leviathan records), but also prevent those races who had developed without Leviathan input from somehow fleeing. Barring all that they would have to keep doing that for a billion years in multiple galaxies, each time giving each galaxy a chance to do the exact same thing ours did. Following each reaping they would have to hunt down each galaxy's version of "Mass Relays" and destroy them/commandeer them to
*sorry my dogs hit my keyboard*
...replicate the Milky Way extermination, move on to the next galaxy and then somehow get back to the Milky Way in time for dinner....which is much more macabre when I think about it.
I don't doubt that they have had a lot of time to do a lot of things I just think its even sillier to think that in the Billion years you pointed out that nothing stopped them...at all and I mean new races in galaxies thousands of years away who might have been freaking Superman lol. Kidding aside the Reapers could have run into the Borg merged with the Death Star at any point in time. Just WAY too many ways that the Reapers could have deemed going out into the unknown pointless or inefficient.
How many races or galaxies would they go through before getting their asxes kicked?
Look, it's all speculation, but the universe just too big, and the Reapers too old to simply hand wave away any expansion of their activities to other galaxies. It just makes no sense whatsoever.
ACTUALLY...the trilogy said that the Reapers return to Dark Space and I believe it's mentioned that they hibernate which would explain why they're so focused on the Milky Way. Maybe they can't exist outside the Catalyst's sphere of influence? SOO there ya go.
Or maybe they can. And maybe they scout other galaxies, and maybe they can create dark space relays to communicate and travel to other galaxies and maybe this and the other thing. But so what? The simple fact that the Reapers are ancient, and that they never went anywhere while massively tech primitive species in the Milky Way (relative to the Reapers) are going to Andromeda just makes no sense. If we can somehow in the midst of our own annihilation put together some sort of inter galactic fact finding mission or whatever, then the Reapers must have purposefully gone to Andromeda during their billion year existence.
Yes yes, i know that they probably have no in the ME story that is getting created. Bad writing is bad writing, but what else is new? Bad writing is old hat to this franchise.