Chinese characters represent the Chinese words or parts thereof, Japanese ones represent Japanese words and parts thereof. That is a semantic difference.
So what you're saying is that because 'chat' in English and 'chat' in French are quite different words with very different meanings, you believe there should be a separate letter 'c' for English and French to enable us to tell those words apart?
It is not logographic, but characters still have meaning - associtated phonemes. Although this is less clear in English, it is emphasized in other languages.
And this mapping is different between languages. So 'c' in English has different meaning to 'c' in Czech.
