Why kanji is necessary to learn ?

Why kanji is necessary to learn ?
I could read one page at one second when I was a boy.
My eyes scan the page, then this information(kanji only) will be transmitted to the hypothalamus.
And my brain re-constructed in to the sentence and understood. But if the information is hiragana or katakana, it will be impossible. Of course, all kanji are registered in my cerebral cortex.
Even if in English, our organs will do the same but will choose main keywords only.