> Why was an above ground design chosen?

Because digging is expensive and there's plenty of land. More efficient to use the budget to build a bigger structure than to build a smaller one and dig down. Bigger structure also gives you better insulation (surface area compared to volume decreases non-linearly with increased volume).

Also, digging in sand is easy. But blowing up granite isn't. The Ice age stripped a way a lot of soil in Finland.