But it's not. It should be, but it ain't.
On one hand, it's not the 5th installation of the series. GTA 1 and 2 are top-down games with nothing remotely close to what GTA3 and its successors are. Yes, same general idea and all, but far from the same game.
So one would assume that there have been three identical games. GTA 3 to 5. But no. There's GTA III. Then there's GTA Vice City, which isn't GTA IV as one would assume. Neither is GTA San Andreas, GTA Advance, GTA: Liberty City Stories or GTA: Vice City Stories.
No. GTA IV came after that. So it's GTA III, GTA $city[5], GTA IV and now GTA V? Heck no, that would be WAY too simple and straight forward. We have to squeeze a game for some handheld bullshit console in between, and please apologize when it wasn't even important enough for me to read anything but the headlines in Wikipedia concerning a game series I hardly have any interest in.
But it seems you didn't even bother reading the Wikipedia headlines, so I guess I'm still ahead.