They did. But resistance is not infinite. It was a really bad earthquake and tsunami combo.

A big tsunami like that happens about once every 500 years. It the life of the nuclear plant is 50 years, the risk of a disaster happening was 1/10. I don't think that this was an acceptable risk. Even a risk of 1/100 or 1/1000 would be unacceptable.

That means that the nuclear plant should have never been there, or should have been prepared to be run over by a tsunami...

Would it be so much more expensive to build a wall 1m taller?

Or the walls 1m taller, or build it underground, or not build it at all. But it does not make sense to build a nuclear plant saying "yes, well, maybe it will be ok"!

Wasn't the problem that the power mains coming in to the station got cut? Apparently the power station needs external power feeds to maintain the control machinery?

The batteries were also flooded.