In Starcraft 2 all the air units seem to not be on a grid with space as ground units are. For example if you have a grid for pathfinding, 1 unit takes up 1 square, bigger units take up 2x2 squares etc. For some reason all the air units in the game dont follow this rule and can move almost on-top of each other to give such a huge advantage over ground units. Does anyone know why they did this or why they didn't put the collision detection in?

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Its not the biggest advantage but it basically means that all ground units have to wiggle forward until they are in range to shoot, therefore since the aircraft can be at one single point in space, they can all attack at once instead of just the front of the army starting to shoot while the back of the army moves forward.

It could be as simple as - it was like that in SC1 and the game would feel wrong if they didn't keep it.

Could also be to represent the fact that there's lots of vertical space.
E.g. When terrans make two bulidings swap positions, they currently fly through each other before landing.. If they had collision in the air, they'd have to circle each other, which would take much longer.. Or, if you could control the vertical height of each, it would be virtually the same as it is now.

Could also be just because air units looked weird wiggling past each other, so they took their collision out.

Remember that to make two units stay on top of each other, you've got to give them a dodgey patrol command (otherwise they'll spread out).

This is the same as in Starcraft 1 and Warcraft 3.

Air units have a mobility advantage, obviously. However, the tight packing is actually a *disadvantage*, as splash damage will decimate those units (e.g. anti-air Thors). Good players learn to spread their air units when attacking in order to avoid this.

Often things are not as simple as they appear at first.

Air units "stack." Ground is 2D, only 1 unit can be in one spot. However, one air unit can fly above another air unit. It'd be confusing as heck to do this in perspective 3D (or more likely, they couldn't in SC1 and wanted to keep the feel), so this is done as "stacking".