Yes...In a fantasy setting, we can come up with all manner of rp justifications for the DM having the monsters doing just about anything...we get it.
But could != should. Having the monster act on knowledge he shouldn't have just because he can and then retroactively coming up with an RP reason for it is, imo, not going to be much fun for anyone. It just leaves players feeling like the DM is trying to win.
It's not that bad to give the players the sense that you're trying to win, if you can pull it off,
while actually not winning all the time. Because, really, the DM decides when the players will win or lose, but the game is more fun if the players feel like they're actions are directly responsible for their PCs' victories.
But that is not really what I was talking about. The question was asked why is it an edge for the monsters to have group initiative while the PCs have individual initiative.
It's clearly an edge, making it easier for Team Monster to coordinate their actions. But, the DM is running a bunch of monsters and adjudicating all the PCs actions, so he's /harried/, while each player can focus on making the best tactical decisions for his character - so, in a way, it kinda works out.
My suggestion is that in that circumstance the DM blunt that edge just a bit by not having the monsters focus fire on one hapless PC while all 3 other PCs stand around with their thumbs up their butts. The RP justification is the monsters are acting as if the PCs CAN intervene...because as far as the monsters know, the big guy with the sword will probably object to them tearing apart his little friend.
Nod. That's the sort of a gentlemen's' aggro that was a staple in many campaigns back in the day - the melee-bruiser-type monsters would preferentially attack 'the strongest looking' character in the party. It saved the casters (tactically the prime targets, and vulnerable), and kept the fighters from feeling useless - put-upon, often dropped, & occasionally dead, but not useless.
But as a DM that his been playing for a very long time (going on 40 years), I wouldn't recommend making a habit of that sort of thing. I don't think players generally find it interesting to have their PC chopped to bits without being able to do anything about it.
Apparently it's more important that the fighter be 'simple' than have actual mechanisms to protect his buddies like he's supposed to, so, yeah, we've been compensating for it the whole time.
"Been a fortnight since we et a halfling..." growls the biggest of the orcs, drooling on himself as he looks to his comrades. "The orc what lands the killing blow gets to suck the eyes out 'er skull!" The ranging scavengers all begin staring hungrily at the halfling rogue before making a move as one toward her. Let's roll some initiative."
This is good because it telegraphs the monsters' tactics, the player's may have a chance to act on that, and the gang-up won't come as a shock...