The top contestants were employing strategies that required APM an order of magnitude higher than what players could do. For example, in brood war, SCVs can repair any Terran building or vehicle but it is not worth the effort to repair goliaths or tanks for a player.
However, the AI could easily manage repairing it's tanks which makes the Terran army much more cost effective.
In SCII there is auto repair and other improved UX that probably limit the ability of novel strategies like repairing vehicles.