Because I want to feel the words and the rhythms in my mouth. To hear them in my voice.
I recognized this whilst I was sitting in the dojo whilst my child took his martial arts class. I was sitting there, moving my lips, and grateful when I could snag my younger child, seat him on my lap, and read a couple poems out loud to him before he wriggled off to find a child with a mobile phone to watch.
You really can’t experience poetry by reading it in your head. Not good poetry, anyway.
Also, notice the double-whilst sentence above. I’m pretty sure in the Hoyle’s Rules of Writing, a double-whilst is a trick worth many points.