They don't though. It's speculation, head canon.
You keep calling it headcanon, but it's not.
Zanza: Many thousands of years ago, while the indigenous trolls of the Zul'Aman empire were battling the invading high elf heathens, great magic was stolen from Zanza. This magic was used by the high elves to enchant their weapons and armor to assist in conquering the trolls of the region.
Player: I think I have heard of this magic.
Zanza: Have you? Then you know how weak and pathetic the high elf version of troll magic was?
The high elves could not understand why a people with their magical affinity could not contend with the powers of troll voodoo. This was an inconceivable notion for the pale skins to understand. They were resigned to using the bastardized version of troll enchantments. I believe they called them 'Arcanum.'
https://wow.gamepedia.com/Zanza_the_Restless
According to in-game dialogue the Amani were superior in the art of enchantment to even the Highborne who came to the Eastern Kingdoms.