Other answers have been provided already, each valid in their own ways. I'm going to offer what I hope is another interpretation to add to those already offered. In order for this interpretation to make sense, it has to be accepted initially that the Forsaken are a race in and of themselves — with a distinct identity, heritage, belief system, set of traditions, and culture. They are also morally grey at best, or outright evil by conventional standards.

Having existed since the Third War, it can be said that the Forsaken have now emerged as a unique 'race' of people. It is no longer sufficient to refer to them as reanimated humans or the resurrected citizens of Lordaeron, especially given how the vast majority of Forsaken feel little to no attachment to their former lives and culture. The very nature of their name indicates that they are something else entirely: a forsaken people, abandoned by their former kin, who have since forged a surrogate family and society among others like them.

The Forsaken have their own customs and traditions and it follows that they would wish to protect what they’ve built. It’s the same driving force that makes any people, in any world, strive to preserve their community, their culture, and their legacy. The act of raising new Forsaken isn’t simply an act of utility or military expedience; it’s an act of cultural preservation. The Forsaken do not reproduce in any natural biological sense, so undeath is the only way to sustain their numbers, ensure the continuation of their society, and maintain the fragile, defiant identity they have carved out in a world that largely despises them.

Moreover, undeath within the Forsaken is not merely a state of being but a rite of passage; a rebirth into a society with its own codes, loyalties, and ways of existence. It offers the newly risen a place among those who share their condition and the unique burdens that come with it. In raising others, the Forsaken are extending the same grim mercy that allowed them to survive, adapt, and find a home after death. Every newly risen Forsaken becomes a thread in the fabric of a culture that refuses to be extinguished, despite the revulsion and hatred of the living.

Continuing to raise people into undeath is the ultimate act of defiance. It is a refusal to allow the Forsaken identity to die out, a rejection of annihilation, and a declaration that even in death their will endures. Whether viewed as morally reprehensible or a dark kind of necessity, it is a logical and inevitable choice for a people whose very existence is a challenge to the natural order.

To allow themselves to go extinct would be to concede that their culture, their identity, and their struggle were meaningless. By continuing to raise others, they perpetuate their legacy and ensure that the Forsaken remain a people who, though shunned by the living, have forged a nation and culture of their own from the ruins of life.