Because even though humanity has sapience, it never conquered the biological imperative to "keep growing", and our current planetary Petri dish is getting crowded and full of our own waste that we can't process. Obviously the only solution is to jump to a new one.

There's also a nostalgia and romanticism to the idea of the frontier, and now that we don't have "anywhere" left to go on the planet I think there's a hole in techbros' hearts that they are neurotic about filling, even with bad ideas.