Firstly, because their speed is only 11 (I've always thought that the speed of Barbarian units is insufficient), and secondly, because the Rampart army will have time to block them with Pegasi or Dragons, which have higher speed. Even on Barbarian territory.
And by the time the other Barbarians arrive... they will all be taken out one by one.
Wüst und leer und ohne Segen.
Hier bin ich durch dich gefangen,
Muss für dich durchs Feuer gehen.
Du siehst mich in Flammen stehen,
Glühend will ich dich empfangen.