This principle is not as universal as you are claiming.
Perfect symmetry between PC and NPC rules has a very key issue: NPCs do not have the same gameplay purpose, contribution to the experience, nor (in essentially all cases) length or quantity of presence in the gameplay as PCs do. Demanding that anything PCs can do, NPCs can do, always no matter what, is a recipe for many ills, among them DM burnout, DM-player arms races (e.g. "scry vs scry" behavior and the resulting never-ending cascade of magic defenses and counters), and severe issues with encounter design (because NPC spellcasters strong enough to not keel over from a particularly vehement PC fart will be bringing far too much magic mojo, able to drop an entire day's worth of spells in a single encounter.)
It's a lovely idea. In a purified no-story, all-"come what may", sandbox/hexcrawl game experience, it's even productive, because the whole point there is that there is no arc, no point nor direction, there is only the places you chose to go and the things you happened to see (or not see) there. But such an aggressively "no story, just events" playstyle is pretty unforgiving, and most players are looking for at least some degree of satisfying narrative or at least satisfying conclusions, which requires that the experience be shaped to suit that end, to at least some extent. That doesn't mean this playstyle should be left by the wayside, to be clear, but it should be seen for what it is: an uncompromising requirement that, if followed in full, tends to exclude a number of things players really enjoy about playing tabletop RPGs.