The endings, they can only explain so much about galaxy after the war.
Moving to another galaxy defeats the purpose of preserving player choices since it renders them irrelevant since it defeats the purpose of saving the galaxy in the first place. Not to mention ark theory is built on nothing but contrivances, conveniences and hand weaving which is are some of the major issues in the trilogy. If they want to learn from their mistakes then they should avoid using said issues in their plots.
And we're right back to this argument, the idea that because you didn't like the endings Bioware has to address them in the next game, when the simple truth is, they don't. Bioware can move onto the Andromeda Galaxy and never discuss the future of the society in the Milky Way since they concluded that story. The fact the game is not called Mass Effect 4 should be a major tip off that they do not necessarily feel bound to show you what happens after ME3. They could set the rest of the series in Andromeda, using their past experience to craft a better games.
Not going back to the Milky Way after that tale is concluded respects the original intent of the devs to conclude that storyline, it respects the decisions of the players, and frees the devs from the plight of other game developers which is to make the same game over and over again until they are desperately trying to breath life into a dead franchise (I'm looking at you Ubisoft).
Preserve life
The creators of the Catalyst controlled the Milk Way, and were only interested in ensuring that their subjects remained alive to pay tribute. There's no indication that they sought power/influence beyond the borders of their galaxy which makes sense. Just because your top dog in your neck of the woods doesn't mean there's not something on the other side of the hill that could flatten all you hold dear. So they create an AI and task it with solving the Organic/Artificial issues that keep cropping up in their own backyard. The Catalyst tries many different solutions before arriving at it's Reaper protocols. In that way it's able to preserve life. So why doesn't it go to other galaxies even if it has the means? Because controlling the cycle in the Milky Way is a difficult enough task that taking resources away from the project thst could cause the entire thing to collapse resulting in the destruction of life it was designed to preserve. Unlike the Geth, the Catalyst can't just keep building Reapers it needs something to base those forms on, meaning each time a Reaper is lost, it suffers a significant reduction in capability. Throwing Reapers at another Galaxy that might very well have the ability to take a number of them out now becomes less cost effective than just maintaining it's own borders. At least this way it can concentrate the reaper forces to deal with an external threat if one arises.