"Only because you fail to improve upon what you've already learned," she quips in the absence of being able to shove or swat him for that comment.
She could, but why would she want to? He's just barely managing to miss stomping on her toes every tenth or twelfth step; she's not about to outright disrupt what balance and rhythm he's found in the dance by thwacking him in the arm. Light banter is preferable — and much more them.