All the folks with system mastery are able to do this, yes. No one is disagreeing with that.
The point is that, to new players, or people considering playing, when they pick up the PHB in a book store or ask a friend, hoping to play an incredibly popular fantasy archetype -- look up the word "Witch" on Amazon, for instance, to see how big of a deal it is in popular culture -- they won't see it there. That sends a message -- probably purposeful through 2E, maybe accidental after that -- that this game isn't for them.