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”.
What is EA is certainly not even half of what they actually did. Since the game was to be released this year, they had to work on Act 2 before EA's launch. If they rolled out this thing in EA now and found it didn't work then people would complain that they had been lied to if it didn't come out in full. If Larian introduces the option to choose the size of the team and it is unbalanced at the same time, people will start complaining. It's safer to just not complicate the matter and let people who want to play with a non-standard team to use the mods. The game is designed for cooperation and such games usually have either 2 or 4 players. I have not come across a game that would allow more people to play. As a rule, it's hard to find more people to play so that the dates suit everyone. It makes no sense for some players to control more than one character. Another thing is that the game is also to be released on consoles.