If you watch the opening credits of The Expanse, you'll see the moon is almost entirely populated, yet if you look closely at the Earth shot, Tibet is basically dark.

Even in a work of fiction we can't seem to make use of the entire Earth!

Or we could colonize the southern Pacific ocean. It's virtually empty of human habitation and although a permanent colony will be difficult to establish, it's still orders of magnitude cheaper and easier than Mars.

Just gotta wait another 50 years or so for the free terraforming effects of AGW!