Increasing the default party size is not such a simple matter. I've done one playthrough with a party of 6 on Patch 6, and with this many characters you can steamroll almost every encounter using basic attacks and little else. For a proper party of 6, the entire game would have to be rebalanced, most if not all encounters remade.
Don't get me wrong, I'd like a bigger party as well, if only to see more story, but it's not as simple as "just making it official".
Increasing the default party size would take (some) additional work, but allowing an optional increased party size requires negligible work. All Larian would need to do is include a checkbox in settings "Allow parties of 6" with a warning when you click it "Warning: The game is balanced with a party of 4 in mind. Having a 6-person party might not result in the intended experience."
As Tuco says, BG3 will be released with different difficulty options, so those who want a challenge but also to play with a 6-person party could up the difficulty.
