In this universe humans aren't the first civilization to reach space in the local stellar neighborhood (a bubble of ~1000 light years across). In fact for the past several billions of years civilizations were popping in at a rate of roughly once every couple hundred thousand years to a million years or so. Each went to interstellar age, founded several colonies, then fizzled out over a course of several thousand years (since their first spaceflight) or rapidly went extinct for various reasons (Who said "Reapers"?!). There was no communication or influence between those civilizations. So each civilization roamed the stars in search of the various resources it needs. Some went full-on megastructure building, but assume that time is relentless and in absence of people to keep maintenance, most of them eventually got destroyed and fallen into their respective suns by the time humans arrived, and similarly most of the remnants of these civilizations were lost to time, decaying orbits and geological processes as well.
The question is - why instead of stars upon stars of mined-out systems where no easily accessible resources had left over the course of billions of years and millions of civilizations that roamed there, we see that there are still asteroids rich in metals, lots of ice, oils on planets that can have oils, and so on (Including, what's important, completely pristine Solar System among all of that)? I suppose on planets some geological events might eventually renew deposits of ores, but what about outside of the gravity well? Yes, the space is huge and has a tremendous amount of resources in it, but we're talking billions of years and a very large crowd of those who want these resources, and the new star systems aren't popping up all that often. Ancient humans could find large chunks of raw copper ore just laying around on the ground - that's impossible in the modern world, everything useful was scooped out and dispersed long ago.