I "think" the difference is that we expect an evoker wizard (to continue the example) while not built on exactly the same chassis, to have similar abilities to the pc class.Literally the first monster in the 2014 monster manual has an ability that PCs can't obtain (Dive Attack).
Monsters/Creatures having abilities that players dont is fine. (there was a 1e (?) fire hawk that basically shot fireballs but it was an ability, not a spell)
Many many of my NPC wizards have abilities that the PCs dont. (This is fine and expected) If players question it I tell them if they want to spend their lifetime researching special abilities like the Dark Lord did, go for it. Might take longer than you think, or might be feasible.
But if every time I grab an Evoker mage stat block they all have Arcane Burst...not as fine and feels off.
Sure I can fix it in the story/home game, but its created a disparity that wasn't there. Just so Counterspell wouldn't shut them down.