The answer is complicated, but it has to do with the feeble ‘phoneticity’ of Chinese characters, as opposed to other scripts that were specially devised to convey the sounds of a language. In writing systems whose symbols represent phonetic information, there is a ‘virtuous circle’ in which the four functions of language—speaking, listening, writing, and reading—are mutually reinforcing.

提笔忘字
https://globalchinapulse.net/character-amnesia-in-china/