Actually, sheet music does show finger positions. I guess there is good and bad sheet music. You will see the number from 1 to 4 in circle next to a note indicating the finger of the left hand to use (1 - index, 2 - middle, 3 - anular, 4 - pinky). But it doesn't tell you all the fingers on ALL the notes. The reasoning here is simple: Once you have the starting finger indicated by the number in the circle, the rest of the fingers follow in a natural progression depending on the notes, UNTIL there is a change, at which point you will see another encirled number on the note.
"Great things can be done by great sacrifices only" - Swami Vivekananda