Actually I can because that is not how the ghost things work. You have to decide when you first die to become a ghost, and Sirius and James did not. Also being a ghost is considered an act of cowardice.
Learn how to make this easy spring inspired strawberry smoothie foaming whipped salt scrub recipe! It's made with real strawberry powder which is rich in polyphenols and Vitamin C and has natural skin soothing properties.