The flood plain of the Mississippi has some of the most fertile, easiest to till land in the country. People build there for because it’s desirable.
But there are risks as well. The Mississippi floods. Sometimes it floods a lot; sometimes just a little. You never know.
When you subsidize building in the flood plain, that is to say when the federal government provides flood insurance at less than the cost that would be demanded by private companies, it encourages people to build in the flood plain.
That isn’t fair. Practically by definition those who don’t build in the flood plain are subsidizing those who do.