Three reasons:

-No one wants to pay for a hard site with structure that "might" be used and sits idle for some length of time.
-No one wants to run for reelection after having voted for a "FEMA relocation camp" with the all the tin foil hattery that would be associated with it.
-No one wants to let a hard site sit idle, so they'd turn it into a halfway house or a homeless shelter, so when the time comes, it is going to be be full (and probably trashed).