We solved starvation. It was actually the last problem to go, despite how the technology had been around for ages - it was a matter of distribution, that was all. We'd worked out diseases, crime, climate change, et cetera, and now the final problem had been cracked; nobody needed to go hungry. We were done.
And, being done, we started questioning ourselves.
What would we do next? What else could our species possibly accomplish? We focused on entertainment, for a while - but it grew stale. We figured we'd work on technology, but without any real incentive to better ourselves, we invented nothing of consequence. We thought, perhaps, that we could focus on exploring the stars... but then it happened. We don't know who started it - someone, somewhere, just had an idea, and it spread like a wildfire.
Our civilization had gotten bored. It needed a hobby. It needed, desperately, some means of occupying itself, and so it invented one. It could have been anything, the Project, but this is what we chose. Who knows why?
It wasn't ubiquitous at first - not everyone joined the fad - but over time, over the course of generations, it began to change. It was the grandest project yet - the grandest project ever - a complete reinvention of ourselves. Every single member of our species, working together on an all-encompassing, planet-wide effort to change ourselves - not into something better, just into something different.
Centuries later, the travellers arrived. By then, we had forgotten. Only the Project remained. And so, when they arrived, we greeted them as we had always greeted, spoke as we had always spoke. As had always been our custom - as long as anyone could remember.
"Howdy, pardner. Welcome to the Wild West Planet - yee-haw!"