When people put things like houses, businesses, and roads in flood plains or use them as make-shift retention basins.

It's bad at 10" or so or rain. https://radar.weather.gov/radar.php?rid=LTX&product=NTP&overlay=11101111&loop=yes

The noaa.gov rainfall total's usually not far off, but was confusing for a moment. All the "3-5 more inches expected" sounds like it's over the same spot; the locations keep moving, so the total's often under 10". The total will change, the link won't, so take that into account when commenting on this post. ("But the map *clearly* shows that it's now over 15"!" Yes, but we'll have different nows.)

Imagine what it would be like at 36". Or 40". Or 50".

Now imagine that you got 35" and upstream of you also got 35". At least around Houston there were levees (making it better or worse, depending where you lived), but still ... Drainage basins filled rather handily, concentrating the runoff. Navigating Houston after Harvey almost required navigation with actual boats.

It's not the immediate hardship that's the problem for most people, however traumatic it may seem. It's the longer-term effects, whether the next 6 months or the economic impact on people that'll still be felt in a decade.

or was it a dirt dam in Houston...hmmm


which still does not negate your point of turning flood plains, bayous, sand spits, etc. into expensive housing developments.

The term "500 year flood" is essentially meaning less now, owing to increased rainfall accumulation from more intense storms.
That fact has been a warning for some years now, but the average Joe Blow wasn't likely to see it on the evening news, esp. Fox.