Some warlocks become necrolytes but only the most powerful ones do. Sargeras helped create the first ever necrolyte and then later on Kil’Jaeden created the first orc ones and those were way more powerful than any of the warlocks and were also not necromancers. Necroylytes did a lot more than just raise the dead. Orcs and humans were the first ever ones. And mostly those were the only ones.