I'd also love to hear from an expert, but my understanding is that building codes make some accomodations for a general expectation of flooding (like no basements allowed), but the region has a huge safety net in FEMA and other disaster relief funds, and so for any given house it makes more financial sense to build more cheaply and then use relief funds to rebuild when the disaster actually comes.

This likely is making for a good deal of misaligned incentives, where public disaster relief expenses could be much less if building codes were raised in these areas, but such change is likely to be slow in coming and we should expect the status quo to continue for some time.