Because it is the DMs Job to interpret the Action the players are taking. That is a necessary part to adjucate.
Without interpretation you reduce the DM to a number cruncher and could replace him with a computer.
I just don't see how this is true.
The complex mental life of a person is subjected to a very large host of pressures exerted upon it and those pressures aren't remotely limited to the needs/wants of the person they're engaging with or the navigation of labyrinthine social cues/signaling, or the multifactorial context of the situation before them. They include wrestling with their own general emotional state, weariness/level of contentedness, their own limbic system, and any number of immediate pressing issues (poor sleep or an argument with a friend or partner the night before or a logistical problem, such as an expense that can't be predicted or budgeted for, that is weighing them down).
GMs having to simultaneously index NPC dramatic needs and index the mechanical result of a move made by a player (both the gamestate change downstream of that move made and the nature of the move within the imagined space) is not a reduction in GM mental overhead. Its a different type of GM overhead than GM decides based on indexing nothing other than their gut/notes. And it is very much a difficult skill that must be cultivated with rigorous practice and time "in the lab."
And the play result of that simultaneous indexing (the bolded above) can be causally linked to any of the rational or irrational factors listed above. Fully-formed people make good decisions for the wrong reasons and bad decisions for the right reasons and make wise decisions that are actively harmful to their own interests and foolish decisions that are actively or incidentally helpful to their own interests all the time. GMs being constrained by having to index dice results in their handling of NPC responses and churning out any of the above is neither "defanging GMs" nor "reducing GM overhead" nor "reducing GM skill in managing their role in play" and its certainly not implausible for people to be co-opted by external forces (including "internal forces that are external to their conscious self") in their social interactions.