Because Farseer is already the Class Title for Shamans.
If you’re a Shaman and you did your Class Hall stuff, you’re a Farseer.
But if you choose the Farseer Hero Spec, you’re a Farseer.
So this creates some weird situations where not all Farseers can be Farseers, and some Farseers may choose not to be Farseers, and maybe there will even be some Farseers who never got to their Class Hall so they never became Farseers, etc.
My solution to this is, shaman keep the farseer title until they put a point in one of the other hero trees, and if they do that the Farseer title gets replaced with a spec appropriate title.
Actually, let’s give all the hero specs an appropriate title
It’s like a doctorate. They gave you honorary Farseer-hood so that you could lead the class hall, but now you’re putting in the hours hitting the books so you can earn it for real.
Slayer is also the class title for Demon Hunters, but a hero spec for Warriors. So the characters who can have the title Slayer and the characters who can use the abilities of Slayers are completely separate.