Why isn't Coca-Cola forbidden, despite its containing potentially harmful substances (such as phosphoric acid), its manufacturing from plants from which forbidden substances are extracted (like cocaine and caffeine), and its belonging to companies affiliated with hostile entities?

The basic principle concerning things is permissibility. It is not permissible for a mufti to prohibit something that is originally permissible unless there is definitive proof. Prohibiting what Allah has made lawful is not a trivial matter. There is no reliable evidence to prove that the aforementioned drink is harmful, and therefore, it cannot be prohibited. The ruling is that this drink is permissible. Whoever wishes to abstain from it may do so, as the matter is broad.