Why can't we just "colonize" Earth?

Update: My apologies: I thought I was entering details but they somehow turned into an "answer". What I meant to say was: A lot of people say we must colonize other planets in case the Earth becomes uninhabitable. But all other planets in our system are also uninhabitable, which means we must create habitable... show more My apologies: I thought I was entering details but they somehow turned into an "answer". What I meant to say was:

A lot of people say we must colonize other planets in case the Earth becomes uninhabitable. But all other planets in our system are also uninhabitable, which means we must create habitable environments on those planets to live on them. But if we can develop the capability to do that, why can't we just use that technology to create habitable environments on Earth?
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