Keeping the same name for all the games adds a sense of unity, and it's good for Square's reputation too. Back when FF2 came out, people were probably like, "Oh look, it's another Final Fantasy game. I remember the first Final Fantasy was really good, so maybe this one will be too. *plays* Hey, I really like it. I'll keep an eye out for more Final Fantasy games in the future." Sure, the same logic could also be applied to the fact that both games are by Square, but giving them all the same name adds a little something extra, and some people don't pay attention to which companies make which games.