They weren’t! Like all natural languages, Chinese languages became what they are today by evolving naturally over time. Their complexity wasn’t a decision; instead, it emerged as a result of centuries of linguistic development, gradually changing over time.

Languages are not designed, rather they end up being the way they are in a purely arbitrary manner, but basically Chinese Hanzi are hieroglyphics that end up moving forward towards modern times.

I take it you don’t speak a tonal language. When languages are developing they start running out of short words made of one or two syllables. They need more complexity within their sound to describe new concepts. For some languages like English, they added conjugate syllabes to words to alter their meaning, ex: run, running (active), runner (one who runs). Another option is to alter the way a word is pronounced, by adding variability to the voiced sounds (vowels mostly) to alter the meaning of a word, the most prominent example is the tones in Chinese languages. Some languages alter words based on a system of gender. Some languages alter words based on respect or status. Some languages add full particle words to help convey different meanings.

Nobody designs a language, people just start needing to convey complex information in new ways and others start to pick it up, there’s a dialectic effect as ways to communicate stick around or get dropped because of a simpler alternative. This is why Mandarin has less tones than “natural” Chinese languages, because it was a simplified language that developed between speakers of different Chinese languages.

English has gone through similar simplifications as people of Norse, Frank, and Anglo backgrounds started to inhabit the same spaces and developed a common tongue. As an English speaker Chinese can be quite complex, the Chinese feel the same in reverse. In truth both languages have similar amounts of complexity, they’ve just evolved different ways to express complexity.