If you live near these reservoirs, you drive by them daily or at least weekly, and see them turn into giant lakes after a mild storm (which for us can mean dropping several cm/in in the course of an hour or even minutes), and are also aware that we get big storms from time to time.
Thinking you are not at risk of flooding anywhere in the area, or near these reservoirs especially, is basically living in denial.
Of course, hindsight is 20/20, but it doesn't seem like there was anything in the way of due diligence to avoid choosing this neighborhood. I certainly believe however that realtors and the local government said that everything was fine in pre-Harvey times, as both had something to lose with saying the opposite. That's why I never rely on those sources alone.
There's a lot of additional information to be gained from Google Maps and OSM, satellite views, walking in the environment, checking historic news, talking with residents, etc. before settling on a neighborhood.