"why not more? Why not make a dedicated instrumental case, or a locative that isn't a sewn-together monstrosity comprised by other cases?" - the sounds of a mind deranged by synthetic languages

Russian has six! Also the rules for declension in Russian depend on, among other things, whether something is animate. Are dolls animate? (Yes.) Are corpses? (Depends which word you're using.) Are bacteria? (Yes if you're a biologist, probably not otherwise.)