Why is Final Fantasy 8, on here? I can understand 13-2, or Dirge of Cerberus, but the main line of Final Fantasy are not sequels of each other.

Because a lot of idiots think Final Fantasy started with 7, probably. I remember reading a review of Final Fantasy 9 where the amateur writer wrote positive things about the game, but then went on to criticize their decision to give cloud a tail, and complain about the lack of explanation as to where the other characters from FF7 went

They are sequels. They may not share characters, worlds, or stories, but they share a common name and many elements that form a lineage from one game to the next.

While the games don't always have much to do with each other, they do count as sequels, the hint being in the numbering (Final Fantasy 3, 4, 5, 6, 7, 8 and so on...)

The Call of Duty games are the same way, with (some) of the games not having a connection to each other but still being sequels. Of course this all comes down a technicality and I can see why you would make a case against it.