Actually, we did, it's just mostly been abandoned because "Accent marks scary," or something. If you look in older texts, you can find words written like coördination, coöperation, etc, to distinguish between two identical short vowels, meant to be pronounced separately from the long vowel in words like soon. I want to say the New Yorker, or some similar periodical still prescribes its use in articles they publish, but that's about it these days.