Because it's not cut & dry, there's hardly any piece in MC / Ony that lasts for the enterity of Classic, they however aren't all replaced by BWL gear.
If you farmed Savage Gladiator Chain / Hand of Justice, you have it and there's no need to go back and farm it again.
If you farm Item X on Retail from an M+, you have to farm the the same item with higher Ilvl again next season.
If you farmed a faction to exalted for an item, you have it and there's no need to go back and do rep farms for that faction again.
If you farmed Torghast for a legendary item, you have to go back next season / patch to upgrade it.
Classic phases aren't seasons, because they don't invalidate the previous one.
People didn't go back to BRD and farmed an Item again because BWL came out.
People also still farmed MC, despite it not dropping [Season 2] loot.