Why? Is there shortage of other habitable land?
There is no shortage of habitable land in TX. Wealthier/High ground areas are mostly occupied or already built out for decades. Outside of erosion of these areas, mainly floodplains offer opportunities to expand towns with commercial and residential structures. But floodplains are an inherent risk and occupants assume that risk all over TX.
Also:
Much of the low risk land around there are privately owned ranches.
Demand for housing and business allow developers to acquire land for a small premium and sell rent structures for competitive market rates.
Overall, flooding is simply a known assumed hazard residents must endure.
Most insurance companies charge insanely high premiums for areas in risky flood zones. Obviously it’s going to be risky when you’re adjacent to a river