Where do you move people out of "at-risk" areas for tornadoes? Essentially, the entirety of the central and eastern US are at risk for significant (EF2+) tornadoes for at least some portion of the year [1]. And then the places that aren't at risk of tornadoes have their own substantial risks for natural disasters-- earthquakes and fires out west, hurricanes and flooding closer to the coasts, blizzards and winter weather up north and into the Rockies, water supply issues in the southwest, etc.
That's why we have FEMA, because "just" displacing people after a natural disaster doesn't work.
Luckily tornadoes are not actually that destructive on a broad population scale. Out of ~1000 tornadoes per year, there are only about 80 deaths per year on average [1] and the national cost of the damage is around $10-$14b per year [2].
Hurricanes are a lot harder to talk about deaths since hurricanes are so big and cause such widespread damage (although Hurricane Maria is supposedly responsible for around 3,000 deaths), but the economic impacts are much, much larger. Just one hurricane can cause hundreds of billions of dollars in damage [3].
I'm absolutely not arguing in favor of getting rid of FEMA, but there are definitely the costs of some natural disasters that could be avoided by relocating population centers. Although tornadoes are not one of them, in my opinion. The cost and death toll just isn't that high for a tornado.
That's why we have FEMA, because "just" displacing people after a natural disaster doesn't work.
[1] https://www.spc.noaa.gov/new/SVRclimo/climo.php?parm=sigTorn