I think you’re mixing up post-game content with end-game content here. To make that distinction here, let’s take FFV and FFVI here. The entirety of the Merged World and World of Ruin are optional end-game content. It’s stuff that unlocks once you unlock the final dungeon. It’s true that many vanilla releases of V and VI would be disqualified from having post-game content at all because you pretty much don’t have anything to do after beating the final boss - that is, after the game, or... post-game. Post-game content only exists for FFV and VI since the GBA rereleases, where you get the Sealed Temple, Dragon’s Den and Soul Shrine - proper post-game content that is exclusive to the post-game. Up until X you only really got end-game content, but since XII you’ve had a bunch of stuff unlock after the game is beaten, and pretty much all of the rereleases of I through VI have some form of post-game content.

So yeah, it’s not arbitrary. Post-game and end-game are just two different things that often get used interchangeably. If you wanna say the post-game of FFVI is the best one on the basis of WOR, that’s a fallacy because you’re answering a completely different question.