The only time you have a party of two is after the nautiloid for a grand total of 10 minutes. You can choose to continue with a party of 2, but why would you when the game has been designed around 4? I understand that this case is a self imposed challenge that only a few person make conscientiously, and therefore it doesn't represent what BG3 is.
That's the point. Currently, you can choose to continue with a party of 2 (or 1, or 3), resulting in an unbalanced experience / self-imposed challenge. Why not 6?
It would be a similarly unbalanced experience, but this time with a self-imposed *easier difficulty* compared to a party of 2. While the benefit of using a party of 2 is less micromanagement/annoying companions, the benefit of a party of 6 is being able to watch all 6 party members banter/interact/trigger plot threads.
How would you warn players that anything past 4 is uncharted territory then? You gain companions very easily in this game so unlocking 6 man parties by default would be a recipe for disaster since it would become the new norm for every players.