Actually you can do it. However, not specifically as a personal trainer that does 1:1 training in the gym.
While you can have you select clients that you train in the gym, there are other services that can make you a lot of money. Creating personalized workout plans for people as well as nutrition/meal planning and supplement plans. These don't take as much time and you can meet for 20-30 minutes with clients weekly to assess progress and make changes.
Finding the right balance here is the key.