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.
Which Is a weird thing to worry about, given that:
- none of the encounters are going to be considered final and set in stone at this point in development. - going by their track record Larian is going to include multiple, partially even customizable difficulty levels anyway. -if people want the option even at cost of making the game easier or harder on themselves it’s not clear why they should take an issue with it. - my own experience playing with six characters openly contradicts your last point. - they admitted themselves a lot of people will mod this stuff, so once again why not test the general preference now that it comes at virtually no extra cost rather than going “Fuck what you guys want.It is what it is”.