We're evolutionarily fine-tuned to live on Earth. We love a blue sky, room temperature, certain levels of gases in the atmosphere, and so on. It takes very little work to live here, relatively speaking.
Living long-term in outer space and other planets means either forgoing most of the things that make people happy, or else attempting to recreate them in incredibly incredibly expensive ways that ultimately aren't as good.
It's cool to dream about living under glass domes on other planets... but the reality is that after the novelty wears off after a couple of weeks, I think you'd realize you'd just rather go back to Earth. It's way nicer.
"What about if we ruin Earth?" plenty of people counter. Well don't. It's a helluva lot easier to apply our intelligence to normal prosaic things here like taking care of the environment, than to building interstellar spaceships that can colonize other planets.