How would you move all that water? A fleet of water trucks carrying thousands of gallons (barely a dent)? A series of pipes across hundreds/thousands of miles? Who’s going to pay for it? And then you get into the problem of not actually knowing when/where a flood is going to hit. Yes, there’s flood zones, but a pipe route is going to be very specific, I doubt you could just pick it up and move it wherever. Maybe something would be done for a long-term problem to alleviate an already permanently flooded area, but it seems like such a massively expensive undertaking, I’m not sure anyone would be willing to do it.