Why? Because they've skipped some absolutely essential steps in the process of learning. Fingerings are usually a mess, counting is usually off, and the student has typically ignored every slur, staccato and dynamic marking in the piece. These essential details are so much more difficult to teach (and to learn) when a song is played too fast too soon.
Young piano players often associate speed and agility with accomplishment, and it's common for elementary school kids to strongly favor fast and action-packed pieces over slow and melodic ones without understanding that all pieces, whatever their final performance tempo, begin as slow and melodic pieces.
Take Martha Argerich's utterly amazing performance of Prokofiev's Concerto No. 3:
I can absolutely guarantee that Ms. Argerich put in hours upon hours of slow and hands alone practice in the preparation of this amazing performance. How can we tell? Because her technique, fingering, phrasing, and interpretation are immaculate. A piece that is practiced to quickly falls victim to sloppiness of all sorts that only slow practice can correct. A person must learn to walk before they can run.
So, if you really want to please this piano teacher, come back to your next lesson and play the most accurate, well-fingered and phrased right hand alone I've ever heard. I'll think I've died and gone to heaven.