A pre-Alpha StarCraft screenshot shows why worker units hover

StarCraft’s worker units hover over ground because in the early stages of the game’s developments, the minerals they needed to gather weren’t found on the ground. A screenshot posted to Reddit shows StarCraft units heading out to space.

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In its pre-Alpha development, StarCraft (which was then named Warcraft in Space) minerals were actually located in space. That meant that in order to access them, worker units had to be able to fly. Even when minerals returned to solid ground, the workers remained airborne, and still hover to this day.

from
https://www.pcgamesn.com/starcraft/starcraft-workers-hover