Why are flash floods so dangerous?

Flash flood is a rapid flooding of geomorphic low-lying areas: washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields. Flash floods may occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam, as occurred before the Johnstown Flood of 1889. Flash floods are distinguished from regular floods by a timescale of less than six hours.[1] The water that is temporarily available is often used by foliage with rapid germination and short growth cycles, and by specially adapted animal life.

Because of the sudden flood which, most likely, has not been prepared for.