For inter-terminal connections at DFW, even if the GA for the departing flight sees exactly when and where your inbound flight arrived, and attempted to estimate when you'd arrive at the connecting flight, there could easily be a 20-minute variance, depending on: how far back in the cabin on the arriving flight you were, how far from the arriving gate was from the train station, and when you arrived at the train station platform with respect to the train's cycle. Given that, I can see some situations where the GA is unwilling to hold the flight for a connecting passenger that happens through good luck to end up arriving in what would have been enough time.