That and lives are on the line. No one wants to be the refiner who sells new, lead free fuel only you have airplanes crash and then be involved in massive lawsuits for wrongful death. Maybe the new fuel causes premature engine wear under some unforeseen circumstances like in cycles of hot and cold weather that were difficult to test for. Or maybe the hypothetical crash had nothing to do with the fuel but the lawyers smell a settlement. Either way there’s a lot more risk when compared to a car where the worst case is you may need to pull off the road. Also most cars are not 50 years old and have engines designed for newer fuel formulations.