the ai system in command and conquer is generally pretty terrible.
their defense systems are 'eh just place one of each turret and you'll be fine', they churn out units til the money runs dry, they have an instantaneous reaction time, and therefore, build way faster than you (by the time you move your mouse to the right location for the power plant, they've already queued five)
it really doesn't help that they don't adapt to the situation at all. if you have a base with only one land entrance (the one marked on the map. ai can't do stuff on any map unless everything is specifically marked in the map editor. i know, i've been there.), then by god every unit has to go through there. even if they have like a billion planes, they're still funnel all of them though there.