The fundamental difference between a dance class and an exercise class is intention.
Dance classes essentially teach participants to think about an audience. The intention is outwards focused, typically about increasing views or delighting people who are typically sitting down. Dance classes miss the opportunities to get more people moving, and also participants sacrifice the exercise elements such as cycling through levels of intensity.
Exercise instructors are (or should be) 100% focused on the movement, health, fitness and cycles of intensity. There is no audience or show to prepare for.