Because the game is still officially called FFXIV: Online ~ A Realm Reborn - the patches are just that, patches which add content, but don't replace what is already in the game. The patch titles are just fancy ways of describing the content and thus attract the attention of players. Besides, a new player buying the game isn't going to ask at a game store for the game under whatever patch title it's up to, they're going to ask for FFXIV ARR, so continuing to refer to the game as it's main title only makes sense.

And before you pass judgement FFXI was the same - the game never changed it's main title on the title screen either when expansions were released (although the expansion titles were added underneath, they didn't replace the game's original title).