One immediate reason is our physiological limit (see wet bulb temperature), which is achieved at air temperatures as low as 45 degrees C depending on humidity.

Another is sea level rise. For +14C that's on the order of +30 meters as currently predicted.

And of course such drastic changes don't come alone. Weather patterns will change, food production will be threatened, governments will be destabilized, wars will happen.

We can tech our way out of some of this but who's to say that we will be given enough time and resources. The universe doesn't owe us anything.