Generally true, but there's always ways to make some of it habitable. For example build every road and highway as a river to channel water to floodable areas. There will always be losses in a Harvey-type event but they will be manageable.
Flood losses are dropping steadily here in the US over many decades. That's due to better planning, better forecasts so people can prepare, and most of all more economic strength so that a disaster like a flood doesn't matter to most people. They just rebuild and keep going.
There's also a good reason there are many "100 year" floods. It is because there are thousands of areas that can get flooded that people have now built in as you alluded to. With 1000's of target areas, we should see at least 10 such "100 year" events every year.
Well if Houstonians should not be allowed to build in flood plains then should Californians be not allowed to build in “smog traps” (inversion layer basins) or wild fire areas? Let’s take Amsterdam which has dikes to prevent the flooding; or take Palm Springs/Coachella Valley, California that has “dry” flood channels that sit dormant almost always except for the rare flash flood event. Not all of Houston is a flood zone. Perhaps it needs to invest in a giant grid of flood channels to take rain water to the sea quickly. Just as smog aggregates in inversion layer basins in Los Angeles, flood waters aggregate in impervious clay soils of Houston. But then everyone should have to pay for such a flood channel grid not just the insured because there is no way to mitigate the effects of flood without passing it to one’s neighbors.