Chinese has a huge amount of homonyms, not differing in their pronunciation in any way at all, except in their contextual meaning. In languages like Spanish it's almost enough to map he sound to a written word, and there'd be really few places where the written rendering would be ambiguous. In English it's worse, you need a larger model of the language to e.g. correctly choose between "no" and "know", or "there" and "their", depending on the context.

But Chinese is in another league; the same spoken word may have five meanings, or ten meanings (open zhongwen.com and check out), and you have to build the complete sentence as you parse the sounds, asses its meaning (or several possible meanings, maybe in the context of a few previous sentences), and choose the written word that would match the meaning. You need to carry a lot larger "sense-making" model along with your phonetic, grammatical and syntactic models.