Thomas Shey
Legend
Because if the DM regularly does even basic things like focus fire PCs. Not pull punches. Etc. These are things the PCs have no or very limited defense against. Some of them will die and since they have no in game recourse to prevent it then the loss will feel and be unfair.
The problem is "focus fire" is actually a particular standout in terms tactics without much answer. Most others involve positioning, and PCs in almost any game absolutely have some options there.
(As an aside, while there's always some benefits to focus fire in the "put down one opponent then move on to the next" sense, its only really strong in games where any takedown is dependent on accumulated damage (and yes, I do note where this thread is, just bear with me), but in games where there's strong thresholding effects (i.e. where getting a good roll can kill or disable an opponent even if they haven't be previously hurt) its less clear that's the ideal tactic.)