Because they are so busy playing that they aren't paying attention to the signals their body is sending. When you were potty training and in the time after you were very focused on making sure they made it to the potty in a timely manner. For a while after the child stays focused on it and it becomes routine, then time goes on and they just can't stop playing, next thing they know it's urgent and they just can't make it to the potty in time. My son is 4 and in preschool in the mornings. He has been potty trained for more than a year and a half, but he has come home with wet spots on his pants 3 times in 3 weeks. Not full on wet, but wet in the crotch. I asked him what's going on and he said that he can't make it to the potty in time. He starts peeing as he's trying to get his pants down. Now I remind him every single morning to ask to use the potty before it becomes an emergency, that he has to stop playing and go.
at 10:42 PM on Sep. 17, 2009