1/3 in French

The answer is basically because it’s not - in grammar and language many things ‘just are’, maybe with explanations that are obscure or lost in the mists of time, maybe just for unknown reasons. French uses irregular expressions for - 1/2 (as does English routinely - one-half) and also for 1/3 (even here, English doesn’t use 1 “threeth” but the irregular one-third), and for 1/4 (where in English we sometimes/often use the irregular form - one quarter).