@Redice Because they are already ATAC in all but name.
Giant mechs casting spells instead of using science type weaponry? Check.
Giant mechs using medieval weapons? Check.
Giant centaur type mech? Check. (Toreadore)
Giant mech that can fly? Check (Sylpheed)
Seriously. Just nuke that world and have the next generation "excavate" Eru's goodies and we already have the world backstory for Vanguard Bandits.