The supply would still be extremely limited outside a few markets where French is a primary or main secondary language. Also unless you're paying extra for the French speakers the chances the small pool will be already booked with normal jobs is pretty high.
I'm not saying Uber should become a service for French tutors. I'm saying every Uber driver has a unique value-add and in a large market there will be plenty of customers for those additional skills.
Are you trying to argue that there is zero use for a person with a car who speaks French in a city like San Francisco? I really don't understand what you're trying to convince me.
Why? Just filter on the "Speaks French" tag.