That's not how Japanese works. Yes Japanese uses Chinese characters that's only a subset of Japanese. Japanese sentences are typically a bit longer than English in terms of syllables and written length - their main reduction technique at the language level is omission, rarely efficiency improvement.

- Why's it so hot today? = 6 syllables

- Nande konna ni atsui no kyou? = 10 syllables (spoken colloquially)

What about written length? You can see the Japanese sentence is a little longer here (when the English uses a non-monospaced font). It's also enough to demonstrates the why using kanji doesn't always provide the huge reduction/compression that you're expecting.

- Why's it so hot today?

- なんでこんなに暑いの今日？