Scientists have actually proven this by studying how the brain responds to different teaching methods. An important study published in the journal Brain and Language show why phonemic awareness is better. To explain this, here is a passage from Teaching At-Risk Students to Read: Why Camp Sharigan is Effective:
"We should change from whole language to phonemic awareness. Why? Neuroimaging research confirms that phonemic awareness is better. A recent neuroimaging study by Yoncheva, Wise, and McCandliss (2015) studied how the brain responds to different teaching methods, particularly whole language (memorization) versus phonemic awareness (sounding words out by letter sounds). Their neuroimaging study showed that when beginning readers used phonemic awareness or 'letter-sound relationships' to decode words they could even pronounce new words that they had never been introduced to. Yoncheva, Wise, and McCandliss (2015) also stated that it was better to teach children to sound out the word 'c-a-t' rather than teaching the child to memorize the word 'cat.' In their neuroimaging study, research showed that the brain responds better to sounding words out rather than trying to memorize words. The authors went on to say that teaching method has a direct 'neural' effect on learning to read."
Quotation used with permission from Springer