Multiple reasons. One is that it is cheaper to build these homes. That is not only good for the builder but for those who can afford to buy a "stick" built but not a home built for protecting them from a tornado. Most of these people are very aware of the risks involved in living in these areas. Different people have different priorities and should be able to make that decision without the government telling them what to do. Mobile homes or "Doublewides" are also still available in these areas. If the government got involved with this they would then have to get involved in homes in hurricane zones and flood zones. Too much government would ruin the housing market even more.
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