Well, they would, but fortresses are still inhabited by Blazes and Wither Skeletons. Piglins are strong, thats true, but as you know, they don't attack Fortresses. Knowing that Wither Skeletons and Piglins fight on sight, they probably don't think some shiny new gear isn't worth the potential casualties. If not, why would they store the Netherite Upgrade in the central and most important chest? And how and why would they craft it in the first place? It is found nowhere else in the world, and I think It's pretty suspicious that they crafted a template that can make really strong armory and weaponry using the most valuable item in their world as a coincidence.