Because they’re designed to help you dance better!
Dance shoes are flexible so that the foot can move well and feel the floor – this enables good technique and grounding. They have soles that are designed for pivoting – this can improve turns as well as protect knees and hips during turns. And the soft soles would not mark or scuff dance floors.
Dance shoes are also given a shape and heel style that actually assist with the required body mechanics of the dance.