Why do people want to leave our planet for Mars or other uninhabitable worlds?

On Earth, we seek greener pastures and would never move to a place that’s less greener than our current one. So far, we don’t know of any other world that’s as exciting as ours. So, I’m wondering why one would want to leave Earth in the first place? There’s an estimate that the Earth’s age is about 4.6 billion years and it has supported life as we know it. Why would one want to go to a place that’s totally unsuitable for living in and try to make it habitable? I would love to go explore space, but I would always return to Earth because this is the best place to live. Do we know of a celestial body that got destroyed by a killer asteroid or something like that? Please share your thoughts.