Because it always happens in the good spots. Floodplains have fertile soil. Waterways provide transportation and drinking water. The ocean provides easy international trade. In terms of economics, New Orleans is at the perfect spot.
However, natural disasters like hurricanes just come with the territory. Similar to how many major cities are in the shadow of gigantic active volcanoes. Volcanic soil is fertile. People tend to build in areas where the short-term gain is high. Whether or not building in such areas is a good idea in the long-term is generally not thought about until it's too late.
We build on the coast because there is a lot of profit to be had. That's why whenever things like this roll through, humanity tends to rebuild on the same spot, even though it's almost guarunteed to happen again. They assess the risk and consider it worth the risk to reap the rewards. Obviously eventually the risk comes back to bite you in the ass.