If it's so easy, why don't they just do it? I ask this rhetorically. I suspect its not ultra difficult but of course would take some of their time and they don't want to spend any time on it apparently which I think is kind of a crock since so many players obviously want it.
Clearly they believe BG3 will be better with four characters. The game will be balancing around 4 player party. Giving official support for a bigger party also creates an expectation for it to be of high quality. When players mod 6 player parties it will break the balance. Some might not care about it, or might see it as a worthy traidoff, but that's not something a company would want to add as an officially sanctified version. They want you to play with a party with 4. And they said, that for those who don't want that, it should be easy to mod.