Not too long ago Death and Void were quite closely related. Yogg-Saron is the Old God of Death. The Lich King worked with Saronite, the blood of an Old God. Ner'zhul is summoning walking corpses with his Void magic in his pocket Shadowlands dimension.
Heck, even further back Death and Fel were closely related as well as the Burning Legion had lots of skeletons in their service. Not just Warcraft 3, as that has an explanation with the Lich King and the Scourge being used as a tool, but even before the existence of a Lich King there were skeletons and Death Knights (Teron Gorefiend) in the Legion's army in Warcraft 1 and 2.
Honest explanation is Blizzard is just making stuff up as they go and they retcon older lore all the time.