As parents, our children have friends. They have streams of new friends. We get to learn who they are. In addition, because we drive, we get to learn where they live. In addition, because these new friends have parents, we get to learn their new parents as well. Multiply this by the number of children.
Our children gain new hobbies and games. We get to learn about those, too. Multiply this by the number of children.
Our children are into TV shows that we would never watch, but they wind up being fun to us anyway or appeal to our inner nine year old. As they age, they drop shows and add shows. Add in all those characters to your learning.
Our children go to school, bringing home schoolwork with expectations and terms that they never used when we were in school. So, we have to learn about their school work. Fortunately, one child readies you for all the others.
So as you can see, a parent's life is chock full o' learn.