If the standard is 48 inches of rain in six hours, it is not reasonable. Reasonability is also why we don't have a four mph speed limit on highways, though that would cut auto deaths to almost zero.
Autos are a higher risk than private dams but the reasoning goes like this. Our economy can't function with 4 mile per hour speed limit. We all need higher speed limits than that. We address the risk by making better vehicles, improving roads, and requiring seatbelts. It's a risk we have to live with and as a whole we would never choose to go about our travels at 4 mph.
One the other hand. We all don't need anyone's private dam. While we can argue that we need higher speed limits despite the cost of life ... can we really argue that we need anyone's private dam? What is an acceptable loss of property and life that the public should bear from the failure of private water dams? The only answer to the public at large is "NONE". It's that simple and making everyone prepare for uncommon events is the only way to reduce the numbers to none.
