If those things ARE what they say, and they’ve been around AS LONG as they say, there should be nothing left for them to vacuum up.
They ultimately will all merge in to one huge black hole and then the process starts all over again, with a new Big Bang.......................
That may be the case. Remember, these black holes are millions of light years away. This means what is being observed is the state of things millions of years ago. We cannot know what is going on now; it’s quite possible that over those millions of years, these black holes have cleared most matter from the surrounding volume of space