But what if Paul's letters were were named after their sender rather than his audiences? Would Romans have worked as well if it were called 6 Paul? Or 1 Corinthians if it were 2 Paul? It's difficult to imagine, somehow.
It's a bit like the Bond films. Would On Her Majesty's Secret Service have been as appealing if it were called James Bond 6? Or Die Another Day if it were James Bond 20? I doubt it.