Because you don't pronounce the u in guard in English. And although the name is Italian and you do pronounce the u in Italian, many proper nouns of non-English origin became pronounced according to English spelling rules in English-speaking countries.
If a person speaking primarily or only English had never heard the name said aloud before, I think it would be reasonable for them to assume the u is silent based on the spelling.
Some people pronounce things "wrongly" until well into adulthood as they may have never, or only rarely, heard those words spoken aloud. Common examples include awry and epitome, and if you have no association with the US, how would you pronounce Illinois, Des Moines or even Los Angeles if you had never heard anyone say them before?
Ok. I see your point; but for Americans, we know how to pronounce these words. Next time you're stateside, I'll teach you New York slang.
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