Aside from Elder Scrolls Online and Fallout 76, those 2 series were built around offline singleplayer gameplay so in the end modding them didn't effect other people playing the game. Shift to online play and even something like a client side mod editing the appearance of things could be used to gain an unfair advantage against other players. TBH it's the potential for misuse likely being the biggest issue with second place coming in as the inequality caused between PC and console because when it comes down to it. To open that door at all would literally be blowing the dam apart and allowing all the water it held back to wash over the world as far as PC is concerned. To be fair though there are already mods for FFXIV available on PC. A majority of them do little more than edit the appearance of specific items which can only be seen by the client with the mod installed and for the most part don't really effect other players. However that didn't stop someone awhile back from making a crying post that they were supposedly being hurt or impacted by the thought that there "might" be some person on PC running these types of mods that "might" see their character completely naked in game and that to them it was the same as someone spying on them when they're naked IRL...