let's take something very basic, the kanji 人 has two on'yomi pronunciations, jin and nin and there's no way to tell which one to use without knowing ALL of the words it will be used in
日本人 - nihonjin, so far so good 三人 - sannin, okay, I guess there's two pronunciations 二人 - futari, huh? 一人 - hitori, wat
so please tell me how I am supposed to know 人生 (jinsei) from 人気 (ninki) from 恋人 (koibito) from 大人 (otona)
even ignoring the irregular reading for "adult" there's still no way to tell those three readings apart in a kanji as basic as person