Monsters should deal more damage, or have more HP, or just be more of them...
All the re-tweaking and testing, and further tweaking and testing of the already existing content (also act 2-3-...) for 3, 5, 6 party members would be a lot of time. And if they implement it, then people would start complaining about how they implemented it, and why it should be done differently...
Ironically, D&D actually has a sytem for doing exactly this. It's called challenge rating, and if Larian hadn't tweaked abilities and combat stuff so much, they would be able to use that to very easily figure out how to tweak fights. The folks at Larian are significantly smarter than your average DM, I'm stating this as a given. If they'd left themselves in a position where they could lean on the given CR for encounters then they absolutely could have figured out how to make minor tweaks to make encounters suit their purposes no matter how many enemies are in play at the time.
This is actually a good point. It wouldn't even be difficult to up the challenge rating on the fly by adding additional specific CR monsters to an encounter based on how many toons you have. There are calculators out there that already exist that would tell you exactly what to add based on Character level, Party size and the actual difficulty level you are trying to achieve.
