1. One of the critical roles of a fitness instructor is technique. Not being able to correct it is a hazard.
2. There's no clear value for a fitness instructor. Selling phone workouts for $0.99 in the app store is unlikely to be as profitable as 20 regular clients paying $50/week for personal attention.
Ultimately, fitness instructors provide personal services. A few outliers, e.g. Richard Simmons, can brand and sell in bulk as celebrities. But most have to focus on client relations and high touch sales.
Good luck.
