I think the problem is the opposite. We evolved on a planet, so we're only (naturally) suitable to live on a planet.

Once we're able to modify ourselves and our DNA and outrun evolutionary constraints, that might change. Once we're a swarm of small, near-immortal devices, I suppose exploring the universe on large scale will make a lot more sense.

But in the short run, it's fun and interesting to explore the solar system in human form, even given the limits of our current technology. And to not worry about what a small, insignificant fraction it is, because it's still a hell of a lot more than we've seen until now.