Yeah, I've been making that argument since Musk and Bezos first announced their intentions (though I usually used terraforming Arizona as an example as that state has become one of the largest polluters per capita in the US).

But the argument that guys like Bezos and Musk are clinging to is that Mars presents an opportunity to continue the human race in an event of cataclysmic proportions (like another massive asteroid strike, the Yellowstone caldera erupting at a scale we hadn't anticipated, or global warming making Earth generally uninhabitable).

Being farther away from the Sun, Mars gets much much colder than Earth, but it never gets as warm (max temps are something like 60 - 70 degrees Fahrenheit). So it's substantially different than a place like the Nevada desert.

I don't argue that we shouldn't be trying to terraform Earth first and reduce our reliance on positive feedback loop functions like air conditioning, quite the contrary. And I do believe all the rocket launches involved getting people to Mars would likely accelerate warming here on Earth. But Earth and Mars are vastly different places that are very far apart, and I think that's the argument for colonizing Mars.

Also, you should look into Earthships.