[p-e-w]

Here's a far more plausible one: Large-scale space exploration and space engineering are ultimately not worth it, and any sufficiently advanced civilization eventually realizes this. The meme that "civilizations need ever more resources" is a reflection of humanity's past two centuries, not some universal truth.

[throwaway3286]

Maybe they eventually figured out of how to hack the simulation, and have every resource they could possiblity want?