For accuracy's sake. Think about it... are you more accurate when you're airborne, or when you have your feet planted? Plus, they don't need to jump... they have the arm strength necessary to make that distance.
Because that adds another variable into the chance that is to make the shot.. if they practice without jumping they do not have to add the jump into the chance of ruining the shot. They are tall enough not to be required to jump and its not a rule to jump. Simply its easier not to jump
The only reason a player jumps to shoot the ball in the first place is b-cuz they don't want their shot blocked. Every player would love to take every shot standing still, but the only time they can do that is when they are shooting a free throw