It's written, it just doesn't have a standardized written form, it's written informally in standard Chinese character but it's modified from the spoken form in

It's written, it just doesn't have a standardized written form, it's written informally in standard Chinese character but it's modified from the spoken form in order to conform better to standard Mandarin forms, AFAIK. From Wikipedia: "if "pure, unadulterated spoken vernacular Taiwanese" were written exclusively in Chinese characters, with minimal use of Mandarin phrases, over 25% of morphemes would have no character, about 25% would have arbitrarily selected (yet more or less conventionally accepted) characters that are homophones or near-homophones, 10% would be written using characters exclusive to Hokkien, and 40% would be written with characters that have the correct sound and meaning. However, in more colloquial styles of Taiwanese Hokkien, the proportion of morphemes written with conventionally accepted characters would drop even lower than 40%."