Any solar powered device that can't rely on constant direct sunlight needs batteries. In the case of planes they may not be able to generate enough power from solar panels in real time even under ideal conditions. So you need batteries that are charged pre-flight. Batteries are heavy and expensive. And they have limited storage capacity and power output. However there's a lot of research being done to overcome these limitations. Soon your idea may be technically feasible, but right now I think we're not quite there yet.