“Alexander” was an incredibly common name in the Greek world. Even within his own family tree, he was technically Alexander III of Macedon. Using “The Great” was a practical necessity for historians to distinguish him from his father’s predecessors and the dozens of other Alexanders running around the Mediterranean. Plus his scale of impact was absurd! Charlemagne literally means “Charles the Great” because there were many Charleses. Finally, while we usually think of Julius Caesar, “Caesar” became a title used by every Roman Emperor for centuries. It eventually evolved into “Kaiser” and “Tsar”. If you just say “Caesar” in a room full of Roman history buffs they actually will ask you to disambiguate which one you mean