Life on Earth is hard for a lot of people, for a lot of reasons -- debt, lack of employment, lack of opportunity, social unrest or civil war in the more unstable parts of the world. If you can start over somewhere else where it's possible to have a somewhat middle-class standard of living, it might not matter that much that the environment outside is harsh and inhospitable.

If life on Mars turns into a "work 16 hours a day, every day, to be able to buy your daily ration of CO2 filters from the company store" dystopia, then maybe everyone had better stay on Earth, but if we can at least avoid that scenario, then for a lot of people the prospect of being able to survive and have a stable job and a place to live and a community to be part of is appealing even if it's on a lifeless rock.

It's also not clear that Mars will remain totally dependent on Earth in the long term.