Because we already saw the Reapers go far beyond their programming in turning on the Leviathans. They twisted the concept of "preserving" organic life far beyond the intentions of their creators.
In short, they have ignored the concept of implied limits and have taken things to their (il)logical extreme as a matter of course
I didn't say implied, I said implicit.
Since their creators were in this galaxy, we've learned nothing relevant.
Like any good computer, they're following their programming as far as logic will take them. But without examjning the programming itself, or seeing some direct evidence of action taken toward other galaxies, we know nothing about their perception of other galaxies.
You (and others) say that any limitatikns would have been mentioned, but there's no reason to believe that. People rarely point out the specific assumptions behind what they're saying, particularly if they think those assumptions are universally shared (they might not even realize they're assumptions).
A big part of what I do here on this forum is find posts which I think rely on unspoken assumptions, and then I reply based on contrary assumptions, but I also don't point them out. My goal is to get people to recognize when they're making assumptions, and be aware that those assumptions might not be universally held.
If the Leviathans hold some relevant superstition regarding other galaxies, and they think their position is inherently obvious and every other intelligent species would obviously hold it too (and our failure to travel there might be all the evidence they would need, given their confirmation bias), why would they mention it.
It would be like talking to your boss an mentioning, every time, that you recognize the need to respect his authority to keep your job. It's true, and it's relevant to the conversation, but you never say it.
So why would Leviathan necessarily mention its assumptions?