Because a 60 meter high seawall around pretty much all of the country would be a bit problematic material wise alone (you'd have to chop down a couple of major mountains, assuming you have those), coupled with the slight problem of then having to get rid of all the water landing in your nice new bathtub by precipitation.

A seawall isn't just a wall, it's more like a dike if it is to survive and that means it has a width many times larger than it's height.

And then there is seepage to contend with. This is anything but a trivial problem and we have never done engineering on that scale before.

The easiest solution would be to move to higher ground.