It could also just be their resources were insufficient and they plateaued or fizzled out before escaping their own planet was possible.

Or that the distances and difficulty of transporting anything very large or complex between star systems turns out to be just too great of an engineering problem for any life form. The energy required to move a life-sustaining amount of mass between stars is mind-boggling huge and beyond our own technological capabilities for the foreseeable future ( we only anticipate interstellar life is a possibility by assuming our technological growth curve will continue unimpeded and then guessing there's life out there further along tha us, which may not be accurate or possible).

Or that they are indeed spreading but doing it in a way undetectable to our technologies we're looking with and we just didn't come up lucky enough to be directly contacted by them in their spread (yet?).

Or maybe they developed a Matrix-like utopia, a self-maintaining and long lasting support system and stopped outward appearing progress?

Or going in another direction, perhaps we really are inside a simulation and $life_bearing_planet is set to 1.

The hypotheticals are nearly endless and not necessarily due to overtly negative outcomes.

Maybe we are them. We used to live there, and conditions reached a point when we had to leave, and we came to present Earth. Haha.

Plausible, but what would be explanations for sending ourselves back to the stone age instead of carry advanced knowledge along?

Maybe they tried and failed? Maybe they arrived on a sterile Earth 3.6 billion years ago with supplies and huge tomes of knowledge, promptly died off within a couple hundred years of arrival, tectonic movement subsumed their landing ship and all other physical records of their existence here and we're all just descendants of their gut flora?

All of this has happened before, and will happen again.

Can you explain that?

I wouldn't have expected every possible situation to have occurred given the finite history of the universe.

But I really have no idea what I'm talking about, so my request is serious.

It's a reference to the plot of the recent Battlestar Galactica remake.

Good sci-fi material here.