Why would it be there?There is no Rapier in the list.
As a note, the classic D&D list is very anachronistic and cherry picks as well since muskets are oddly absent. It's been more implied than anything else that the D&D equipment list should be curated. If your game is early medieval, plate mail armor should not available. Just like a list of magic items in a D&D book shouldn't be a magic shoppe.