because terran is the race that you need to keep pressuring the whole game while building more production and producing units in the same time.
basically, you need to macro while microing units.
Not only that, but you're dead if your main gets invaded because they can camp your production facilities. Zerg can mass units from other hatches and Protoss can warp in anywhere. If you've lost your main army and they're near your base, it's over because units come out one at a time and from one location. You have to be far more mindful of that possibility.