Because there's a brace period (the ref shouts HOLD!). At this point, you're not allowed to pull the rope, but merely make it taut.
Then he counts down, then shots GO!
On top of that, power = strength and speed. The rate at which they can bring their strength to bear, together, is paramount. Think back to the tugs you watched...were they slow, leisurely affairs?
Of course, not saying one side is slow and leisurely. Relative to the other side? Yes.