We already talked about this in the past, but one of the most bizarre things about Larian simply refusing to consider the option (mostly under the assumption that most of the casual audience would prefer four characters) and Swen saying explicitly "We'll leave that to modding" is that... Well, we are in a goddamn early access.
What better chance there could be to put both options to the test letting players choose at the start and then tracking actual data of what people prefers to do? Especially since as I said it's already trivially easy to "hack" the feature into the game with some minor savefile editing and it would be even easier for Larian to implement it properly in a more orthodox manner.
The answer is simple, they don't want to get into a situation where they will have to balance the game in several different modes for each difficulty level. Neither the schedule nor the budget is rubber. In some places the locations are too tight to allow for a meaningful fight of more characters.