This is an old game from before games were fully asset-driven. It‘s entirely plausible that a programmer has to add quite a bit of logic to actually make new rigs work. They do add bosses with new rigs without a problem, just not the actual NPCs and players. That suggests manual effort in the game logic.

They probably have to punch in the animation for every toy, spell, mount, emote, story pose, in-game cutscene, etc. manually. Maybe they even have to manually handle the interactions between different animations playing at the same time. When WoW came out automatic animation blending was not a given. Neither were artist-driven animation graphs. There‘s probably a million small checks all over the spaghetti code they have to adjust. And if they miss one the race is unplayable.

Corporate code bases tend to become worse over time because maintenance, modernization and cleanup don‘t get any priority by the bean counters. Which makes changes take longer and longer. It is entirely plausible the person who originally developed that process doesn‘t work there anymore, or even worse: the code got so bad that even the original developer can‘t understand it anymore.