In hindsight it is obvious, but at the time of construction, you would have to justify spending money to protect against both a 9.0 earthquake (pretty rare) and a tsunami larger than one ever recorded at the same time. Which can only happen if you have the 9.0 quake right off your coastline.

At the time the plants were built, there was no geologist on the planet that believed Japan could even have a 9.0 quake, or a 30 meter tsunami (which you needed the quake for anyway). Thus, at the time, the plant was over designed for all possible scenarios.