The distribution of jet fuel is not uniform. Some regions are unlikely to experience shortages while others are already rationing. Global supply chains aren't perfectly elastic, shortages are a local phenomenon. Rationing in the US, for example, wouldn't make sense because physical shortages are unlikely to exist there; the US exports jet fuel and has a completely domestic supply chain. Market prices will increase but the product will physically be there.

An airline can only schedule flights if fuel is guaranteed to be available at both ends. If they fly their plane to a part of the world experiencing severe shortages, the plane may become stranded there because there isn't fuel to fly it back.