Originally Posted by
Dracaunt
It's risk management. Sometimes it's too costly to prepare for the absolute worst, because the absolute worst might happen once every few hundred, or even thousand years. Take for example The Thames River barrier in London, it will protect against high tides, but not against the floods that happen in that have a 1/3,000 chance happening (that figure was pulled randomly out of my head, I did it as a case study for a geography module like 2 years ago). It's not built to prepare for the absolute worst because it would cost more to built and far more to maintain. Sometimes it's not economically feasible. Japan is pretty much built to withstand strong earthquakes, but not something of this magnitude.
From your analogy, it would be like you buying a coat in your local shop and then trying to cross the Arctic during the coldest period with it. You don't buy a coat in the chance that a super-arctic bizzard is going to cover your area and kill off a large chunk of the population inhabiting the area. The odds of it happening are in the 1:1000's ratio. It would cost too much for you to buy equipment for that, for something that will probably never happen in your area, maybe never.
They have contingency plans for these kind of things, and we can see them being put in place, they did not have infrastructure to withstand this kind of thing. There was a video of a flood barrier with the flood water just going over it carrying boats like they weren't even there.