It's because this game is not like those games in the 90s, you had either everything turn based or not. Also never cared about formations here anyway. If you see a fight coming you probably place the heroes unchained anyway.
Then why have the formation options in the game to begin with? It clearly works except it unintentionally puts your selected character in the front (which is still an unfixed bug). Also what do you mean it's not like those games in the 90s. The previous Original Sin which came out in 2014 also had working formations. Does that count as a 90s era game too? And what about the fights you don't see coming or what about when combat is surprise initiated through dialogue? There's a lot of times where you can unchain and fight but there's ALSO a lot of times where having your characters in proper formation would help especially if you're playing a mage or rogue or anything that isn't a fighter or knight.
Based off your logic: Don't fix any bugs because it doesn't affect me. It's okay Larian. Don't fix the Tarquin bug, it doesn't affect me.