More or less the whole country is the result of river deposits of eroded material inland as the current of the river meets the essentially static sea, so transportation ceases when the water slows down. Other examples of the same (perhaps with ice age glacial input to a greater degree than in the Netherlands) include more or less the whole of Denmark and on top of that hge tracts of land at every estuary on the face of the planet, including in Britain. On top of this, the Dutch have reclaimed additional land from the sea, all of it flat.
Great answer, Karl. I feel intellectually inferior, and ashamed for my childish answer! By the way, when I write my first novel it will be called "The Essentially Static Sea" :-)
The sea is not strictly static, hence "essentially", because of currents but these are very slow moving. If my serious answer was misplaced, I apologise, but it was not clear that you didn't really want to know the truthful answer and, since you had already had humorous ones, I thought it would not hurt. No slight intended.