Why antarctic research stations are built above ground instead of on the ground or underground?
Antarctica is a whole continent and it is as diverse as any other continent.

There are two reasons buildings are often built off the ground on pillars.

1.- Avoid snow accumulation: Although in average Antarctica is a dry continent, in some coastal areas there can be massive amounts of snow accumulation. Building on pillars spare the buildings to get buried and most importantly allows the wind to blow the snow away. If you build on the surface, the snow will accumulate on the lee of the building. This happens even if there is very small snow fall, because wind transport the snow for long distance and the lee of a building is favourable for accumulation. Therefore, even it snows 1 mm, the millimiter that fell in all the ground 100 km upwind can end up piled behind your hut, and block your door.

2.- To rise buildings: Many of the bases are built on ice, and all those are buit on the accumulation area of glaciers or ice sheets. Therefore, with time the surface will rise relative to the building and will eventually swallow it. Therefore, in many cases (like the Amudsen-Scott base at the South Pole). The pillars are designed to be extended, therefore allowing to rise the building as the ice below thickens, keeping it at an optimal distance for the ground.

Regarding the inquiry about soil temperature, again that is too broad. Imaging asking about the soil temperature in North America, the answer would be quite different for Mexico than for Alaska. The same contrast can be found between bare ground areas of the Antarctic Peninsula at latitude 60°S and at almost 5,000 m of elevation on Mount Vinson plateau at latitude ~80°S