I'm not American, but I do work in safety, and the massive system failure after Katrina and how it only got worse since then is a pretty popular topic in the literature.

surely you still own the land and have permission to live on it? So worst case just stick a caravan there and live in that

The thing is, how? It's kinda like homesteading at that point, but worse. Having a place to live isn't about the square of dirt, it's about all the stuff around it. You need food, water, power and plumbing. If all that is gone, you can't really live there longterm. The kind of cascade failure is what makes the area worse than wilderness, because wilderness has at least some capacity to support people, where flattened suburbia doesn't.

Here the majority of the cost is land that you have permission to live on.

Right, but that land is valuable for reasons. Some of those reasons might include facilities, access to roads and trains, the very appealing property that it doesn't randomly flood, the community and other things.

And that's the problem. You bought the plot, and the house, for say 500k. The house the gone, and the plot is in Schrodinger's suburb, which is potentially valuable, but only if everyone rebuilds. And you already know they won't, because even if you can afford it, not everyone can. And you can't sell it, because who in their right mind would buy land that floods every few years?