You're really not understanding me, frankly.
No, I'm understanding you perfectly fine, but it's incredibly tunnel-visioned rationale towards making this contingency project happen.
Being able to do something doesn't mean that they would actually do it. There's a difference between having the technological capability - which everyone has - and having a reason or motivation to leave the Milky Way.
You're underestimating the power of curiosity and hubris by a country mile, and you're overlooking a billion years of cycles.
For all we know the Reapers are restricted by their programming or the Leviathans' original instructions
Why would they be restricted?
I mean, it's not like they have a desire to explore anything
... they don't?
and their hubris got all retconned by ME3.
How did their hubris get retconned by ME3? What happened by the end of the third game that removes the possibility that their mandate, established in ME1, would take them to Andromeda to impose their order on organic chaos? If anything, their revealed purpose would make them even more driven to purge the universe of their intended problem.
Wouldn't rule the Reapers' capabilities of travel out. Without their interference, Andromeda will have had billions of years of unchecked development (why haven't anyone come to the Milky Way?) and no relays.
The Protheans may indeed have explored beyond the MW, eventually, but they'd only just attained a galactic empire by the time they were destroyed by the Reapers.
If us "primitives" can wage war and build an ark project at the same time, I'm sure those organized and technologically advanced motor-scooters can walk and chew gum, too. They've mastered the art of stasis pods, after all.
My point is: nobody before the current cycle would see a point going to Andromeda, even though they had the ability to do so.
And that's an incredibly erroneous assumption, due to the multitude of reasons why other cycles would do so: exploration, expansion, advanced resources, a contingency home, the works.