There are several answers to that question. One is simply that war is largely about luck and an experienced commander knows that. Another is that the cavalry’s job was in fact not to turn and flank. Rome never had a cavalry tradition and the best that could be hoped for was to chase away the enemy horse. Scipio could count on his infantry to win against enemy infantry one way or another but the enemy cavalry was a wild card that Romans had long found difficult. The Numidians returning and blindsiding the Cartheginians was in fact a bonus. Furthermore for him to tell the Numidian prince how to command cavalry or that his primary mission was to help Romans would go quite a ways against cultivating him. We do not in fact know what he told him along that line but it would have likly had all the awkward compromises involved in coalition warfare. Scipios Numidians in fact likly thought of Hannibals Numidians as their primary enemy and thought of the punic wars as a background for their local power struggles. Furthermore cavalry is notoriously hard to control and there are many instances of it charging to far.
I suspect that Scipio thought of his cavalry primarily as a counter to enemy cavalry which had been so deadly earlier in the war and relied primarily on his infantry which he was familiar with.