[card_zero]

You reckon the perfect information version is cheaper than an algorithm that works only on the tiles near to the unit? But this quickly gets too complicated to discuss; there's the confusing matter of precalculation versus live updating.

[sdwr]

The perfect information version calculates the path once on click, the local version needs to recalculate every time the unit moves (and new tiles are "seen")

[daymanstep]

Recalculating the path every game tick is too expensive. What brood war did was to just have units wait for a second when they collide with something before recalculating the path. This resulted in horrendous "traffic jams".

It's also why Brood War players got into the habit of spam-clicking, because the game would recompute paths every time you issued a command, so spam-clicking resulted in smoother movement.