Personally I am completely alright with a party of four.
Naturally I would want to grab every single companion purely for story purposes as I really like all of them. It would make the story a lot more interesting and fulfilling, but it would also feel a tiny bit crowded and less intimate, for me at least. So personally I am perfectly alright with four and really like the intimacy it brings with chosen companions, as it feels to me like I'm establishing a proper relationship with them by choosing who accompanies me along my journey.
As far as technical aspect goes; implementing a party of six is really nothing complex. The UI would just need some simplistic minor additions and slight adjustments to conform to the size of the party (as I've showcased in a feedback thread not related to this), along with increasing the HP of all NPCs in the world to reflect the party size. The cutscenes would be the only thing that would need some work to include six. So it's not really complex, just would take some work to make it look good.
But it's not about whether they can do it, it's about whether they want to do it. Ultimately Larian has a vision for the game and some suggestions fit into their vision while some do not, because we see only a very small piece of it, while they see it in its entirety and everything beyond it. They have envisioned a party of four. Same as they envisioned that Baldur's Gate 3 will be turn-based. Same as they envisioned that Baldur's Gate 3 will have a chain system. Same as they envisioned that BG3 will not feel nor play as BG 1 or 2. It's simply how they envisioned it, which I personally am completely okay with.
All this is literally done except increasing enemy HP. If you use the mod, you change the max party size value from 4 to 6, and it works beautifully. 6 characters on 1 screen, inventory works, etc. No work would need to be done. Just let players make the call. And most of us who have tried the mod have even said that we don't care if they tweaked encounters. Let us play party of 6 without changing a single other thing. It's a blast and encounters actually aren't that much easier. In fact, I found the game far less frustrating because I didn't feel like if 1 party member fell I was severely handicapped. 1/6th of a party is a lot less severe than 1/4th.