They were originally in groups, though that was a long time ago ( a year maybe?)
Though yes, in ArmA terms, group AI will always outperform singular ones. Group AI have a teamleader to order them to do stuff, which the AI can do very very well. Singular units are rather stupid.
The missions should contain variety of singular units, and small/big groups, so I don't know why it is just singular units now. That sounds like a very strange decision by the mission maker, no offense to anyone in particular.
DaiZy Single-player Developer (AI Units)