Thats part of the answer.
The other part is that the transportation and housing infrastructure for such massive moves simply does not exist.
There are not enough lanes on highways, and the fuel distribution system isn’t capable of delivering the amount of fuel that would be required in such a short period of time to all the vehicles that would be required to carry such an evacuation out.
It sounds like such an obvious answer...until you start to actually calculate real world resources and move from the theoretical to the practical.