It's supposed to help the kids get over their fear of going to the hospital. They think doctors are only around when really bad things happen, or when one of the parents has to go away for a while because they're sick. So playing doctor gets them used to the idea that sometimes you just need a checkup, and it can make you better. It's one of many role playing games kids play.
Then they play doctor, and they find the genitals.
Then they keep playing doctor for years and years and years.
Then they get older and watch hospital porn.
So yeah, tons of people of all ages play doctor.












