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.
That will be a sad day. But my children will cry with me. Joanne K. Rowling was my childhood, and I will cry- literally cry- on the day that she dies, because I think a piece of my heart will die too.<<I'm literally crying rn