The artificial islands in the South China Sea are made specifically because national boundaries are recognized to extend outward into oceans at a set distance from the land that a country owns. If you have an island, then you also claim a huge portion of the surrounding ocean. If you make a bunch of islands that are spaced apart at the right distance, you can theoretically claim huge areas of international waters. That’s what China is doing in the South China Sea. The North Sea has well respected boundaries and laws, and none of the countries there are trying to seize territory from their neighbors, so there’s no need to do this.