The main reason is that the recipient language, in which borrowed words are fully or partially assimilated, may have very different morpho-phonological properties from the donor language (e.g., 'orange' and 'sugar' are not perceived as foreign by native speakers, but these are English words borrowed from Arabic (nArnj) 1 and (Alskr), respectively).
morpho-phonological loanword adaptation is more complex than is typically captured by transliteration or cognate models.