One could possibly argue that undead shouldn't exist as they are evil. They take beings that already have a spirit and warp them causing them to be slightly different then they once were personality wise while also keeping their former memories forcing them to live a miserable existence that not even they themselves want(at least most of them). And once these poor miserable souls can finally truly die they are sent to a hellish afterlife all because someone wanted more people like themselves. They don't even give people the option to become forsaken they choose for them. Had they been going around offering undeath it might be another story but they don't. And this is a world with magic do you really think they could find a way to talk to the spirits of the dead before raising them? So even if the living wouldn't take the offer there are plenty of dead who probably would.
The only other "races" like that are demons and worgen and the only worgen that run around converting people are those who haven't been "purified" of the rage and cannot think or reason properly. And even then as far as I am aware you aren't doomed to some horrible afterlife when you become a worgen.
I don't believe this I was just playing devils advocate as there are plenty of Forsaken/undead who don't go around trying to "recruit" more.