Adaptation costs money, too. Can you imagine how much it would cost to build 3m dikes for every coastal city on earth? Or move the cities uphill?
Then even after you build all your dikes or move the coastal cities, some of your cities get too hot to live. You’ll need to move the hotter inland desert cities, too.
Even after you finish moving Riyadh and Kolkata, you‘re still not done. Every time you add another 100ppm of CO2, your semi-arid cities get too hot, and you need to move them as well.
This is not sounding like the cheaper option.