Floods, fires, tornadoes, and hurricanes only produce grave threats to the individual towns, cities, and maybe some multi-county regions that they cross. For individual incidents, we're relatively prepared beforehand and we know how to respond afterward.
There's no comparison to having nearly everything in multiple states obliterated to the point that rescue takes weeks or months to arrive. We're not equipped to instantly support millions of refugees in the US. Moving hundreds of thousands of people for Katrina was probably at the upper limit of what we can effectively do (for various definitions of "effectively"). A killer quake out west could have 10 million refugees.