And what precisely do you mean by "seeing and avoiding"? Ugo the caveman sees a slowly rising river, Ugo runs to the village, village safely runs away? Erm, no. When you see a flood wave several stories in height, you may have a few split seconds to start videorecording on your phone, maybe the phone will be found afterwards and handed over to your surviving out-of-town relatives, it will make a nice clip in evening news...

With floods, you rely on authorities and early warning systems just as much as in radioactivity, actually you can buy a cheap dosimeters/Geiger counters and there are enthusiast networked detectors - but did you put a webcam with trained CV model on all surrounding flood paths? Did you put chemical sensors around all railway tracks in the neighborhood or do you just as well rely on "seeing, understanding and avoiding" a chlorine gas cloud?

It is just the prehistorical evolutionary parts of the brain speaking - we see flood, radioactivity we don't, danger, quickly bash it with a stick!

Eighty years ago that was the case. Fifty years ago that was the case in many places. Today that is no longer the case. There are very robust and effective sensor networks in any and every location where there exists any possibility of a radioactive concern.

The point stands - for the individual it is very reassuring to be able perceive a danger. And not all radiation sources have masses of alarms near them. Hospitals for example don't put detectors everywhere and they have some very nasty radiation sources.