Reapers are billion years old and travel at 10k times speed of light. They would be in every galaxy in the universe by now with massive fleet for each with no need to move from one to another.
The distance between Earth and the edge of the observable universe (read: observable) universe is 46 billion lightyears. Reapers can fly at around 30 lightyears per day. Assuming "year" refers to Earth years, it would take the Reapers 1,533,333,333 (rounded down to the nearest whole day) days to get from Earth to the edge of the observable universe. 1,533,333,333 days is roughly 42 million years.
And this is only what we can see in space. The total size and diameter of the universe are almost certainly larger, and one must remember that the Earth doesn't sit directly in the center of the universe, so the 46 billion lightyear figure could very easily only refer to the side closest to us, and in fact be far, far larger going the other way.
So, assuming all of the above (because none of this is concrete fact), it would take them 42 million years of (nonstop) travel to reach the edge of the known universe only from Earth. And remember that they come to harvest us every 50,000 years. 1 billion divided by 50,000 is 20,000, and it's known that some harvests can last for well more than a century.
What then, do you propose that the Reapers are constantly flying around the known universe, never ever stopping to refuel or recharge or whatever they need to do to keep their batteries going, so to speak? With all the speedbumps like abnormally long harvests and the sheer travel time between point A and point B, I very highly doubt the Reapers will EVER be in "every galaxy in the universe", especially since stars are always being formed (in a cosmic sense), and a galaxy is, for all intents and purposes, just a mass collection of stars with whatever other celestial debris that is collected along the way.
Even with only what we are able to see, I'm not sure many people really understand the sheer size and immensity of space.