It's because this is a Romanisation system for Latin transliteration in general, not English-specific transliteration, which is actually much more accurate and far more convenient to the hundreds of other languages that use Latin. Much more accurate because English simply cannot distinguish amongst the various phonemes, so writing the four distinct types of "ch" sounds, for instance, with the only English equivalent would be worthless to trying to figure out the meaning (imagine if the English "ch"/"j"/"sh/s" sounds were written with the same letter. "chin/shin/gin/sin" would be indistinguishable).
This transliteration method uses all the Latin letters (except V, although some people even use that to represent the "Ü") to represent every Chinese phoneme without any confusion. As I mentioned before, there are four different "ch" sounds: a pair of palatals, and a pair of retroflex, each pair coming in the aspirated and unaspirated variety. Regular palatal is "j", and aspirated palatal is "q", while the retroflex counterpart is "zh" and "ch". And there's even the regular fricative version of both of these sounds ("sh"): "x" and "sh" (although thankfully no aspirated distinctions).
As for Nguyen, they actually do pronounce all of those sounds. Starting with the velar nasal ("ng"), which is both labialized and palatalized by the "U" and the "Y" respectively. The "E" is the only vowel in that word. It's hard for an English speaker to detect, let alone pronounce all those sounds, but rest assured that there is a purpose for their orthography, it's just based on phonology that is hard to detect and reproduce by many Westerners. And unlike Pinyin (the Chinese romanisation system), the Vietnamese writing system IS their writing system.
· 6 years ago