Because there's been plenty of literature and huge double blind studies backing them. It stands to reason that if you don't trust companies making vaccines, why aren't you at least perusing their output?

Also vaccine availability is how the western colonial project will manifest itself (we already got cute a priori justifications that explain why the first world will receive the vaccine at least a whole year ahead from the rest of the world) not by the effectiveness of the vaccine itself. Sorry, it's a rude awakening and all, I know Cuba is pretty good at vaccine development and medical research but they simply cannot leverage the massive budget a worldwide vaccine trial demands