There actually were such dikes in some places, you can see them overflowing in some of the videos. The thing is, you have to decide how high a dike you're will

There actually were such dikes in some places, you can see them overflowing in some of the videos. The thing is, you have to decide how high a dike you're willing to pay for, which is a cost vs. remaining risk tradeoff. The risk of the 5th strongest quake ever recorded occuring right before your piece of the coast was apparently considered acceptable - Hindsight always wins.