Why does Chinese Pinyin have many letters with the same sound but different meanings?

Chinese characters are typically monosyllabic. Since there are far fewer syllables (about 1,300 in Mandarin Chinese) than there are characters that are commonly used (about 5,000), the degree of homophony in the language is very high. Moreover, about 80 percent of Chinese characters consist of compound characters made up of two components, one of which (the signific) provides a categorical cue to the meaning of the character; and the other component (the phonetic) may cue in an all-or-none fashion the pronunciation of the character. However, the phonetic parts in these compound characters are themselves characters, having their own pronunciation (thus providing phonetic cues) and meanings (typically different from the meanings of the characters in which they are embedded). The cues they provide to pronunciation are neither constant, as some may function as the phonetic part in certain characters and the signific in others, nor reliable, in that less than 30 percent of the phonetic parts provide the correct pronunciation for commonly used characters. In other words, a notable property of the Chinese script is that is has an opaque script-speech relationship.

Furthermore, being constructed following the logographic principle, Chinese characters are basically morphemes, and more than 60 percent of Chinese words are comprised of two or more characters. Since the majority of characters can join others to form multiple-character words with drastically different meanings, the meaning of a character can be highly context-dependent.