Because they take into account not only the development, but also the production constraints, which is not just turning a dial. That said, approaches are kind

Because they take into account not only the development, but also the production constraints, which is not just turning a dial. That said, approaches are kind of different compared to traditional vaccine development. If efficacy is demonstrated, there will be a huge pressure to scale up production to get it around ASAP. That's why institutions like the Jenner Institute in Oxford (which develops one of the vaccines further ahead in the race) is seeking partnerships everywhere (at risk, because for now no one knows if it works). No single entity can make billions of doses. P.S.: All these measures and reasonings seem to forget the eventual availability of effective pharmacological treatment, which might reduce the medical impact of this disease.