An AI would react to the transport as some data item in its to-do list and not recognize the threat of drop UNTIL it landed, just following the script and being purely reactive. A human player is proactive, he doesn't follow 'the script' and adapts to real-time events, even changing build orders to adapt to enemy army. These bots micro is geared to just win local battles(mini-scripts), not to read-ahead and prepare.
In tactics and micro, despite having higher-APM and reaction speed, humans have more nuanced understanding of tactics depending on terrain/unit positioning, while bots rely on brute-force and simple repeatable actions, limited by their programming. Unlike complete information on the board, RTS forces these "rapid plan changes" where things occur 'out of the script' and AI has no idea how to fight it, making mistakes and losing resources. AI is blind to future threats and implications of build orders, while humans always send spies and try to predict enemy future plans. Knowing that enemy has 3 expansions instead of 2 means alot to human player, while AI wouldn't see when it loses attack opportunity window against weaker expansions(due less army units).