It also means changing every single fight by either increasing the number of enemies you fight or increasing their HP pools/number of abilities to keep the same difficulty. So yes your party does more damage but the enemies would as well do more as well as soak more. Net effect is longer combats with more turns total as i said. It sounds like you have never developed/balanced games so ill let it slide
Not to mention this late in development if they have been balancing every encounter for a party of 4 then suddenly changing to six is not going to happen.
No. This has been gone over repeatedly in this thread. Larian could simply modify exp gain per character, with the simplest method being dividing exp equally between all participating characters. A party of 6 would be lower level than a party 4 of, auto-balancing itself. No need to change any encounters. Larian can continue balancing combats for a party of 4, and parties of <4 or >4 would rely on the exp and level difference.
It's also not that late in development. Larian will have to do a final change to ~all encounters after they settle on final mechanics (e.g., removing high ground Advantage likely changed a lot of combat difficulties), so it wouldn't add that much work to need to add an enemy or two to the fights. Copy-paste enemies.
Edit: Furthermore, many people are simply asking for an option of a 6-person party without changing anything else about the game. So by definition, Larian wouldn't have to adjust encounters to give us this suggestion.