Because, quite simply, the court doesn't care IN THE LEAST about where the parents live. You can go live on the far side of the moon with Newt Gingrich for all the judge cares.
But the court DOES care where the child lives. If the child is established in a location, the court may get involved if either parent tries to move the child without the parents agreeing (in some states, the court needs to be notified even if the parents agree).
Furthermore, visitation for a non-custodial parent is a right and not an obligation. NCP doesn't have to visit, so if NCP wants to move to Hong Kong and never see the child, that's their choice. OTOH, for CP, you have an obligation to make the child available for court-ordered visitation. If you move without permission, you probably can't do that and are therefore violating a court order.