In my observation single kids have a higher tendency to indulge in role playing and making up imaginary characters and games. Inclination to do so could stem from the fact that they do not have siblings to play with and may not be adequately engaged outside of school. In comparison kids who have siblings or those who live in a joint family are likely to be involved within themselves though they too would be making up games and characters.
I do see this with my son indulging in creating characters and, at times, a complete script of his favorite cartoon / fictional character is developed and enacted. A similar trait I have noticed in kids who have sibling but the age difference between them is significantly high.
Amount of time parents and family members spend with the child and the level of engagement directly impacts the propensity to role play.
As parents, it will always be our endeavor to spend / maximize the time we spend with our children and keep them creatively engaged but at times there are days when this cannot happen.
So the larger question here is that as parents shall we ignore or discourage such role playing? At what age are they likely to outgrow this?