I think there are three basic reasons why pedopihles exist:
1) some people are simply hopeless cases
2) some girls really look better than porn stars nowdays
3) some people didn't play with children enough when they were children themselves (either due to misconcepted society or boys played with boys only and girls only played with other girls), and then they feel lack of socialisation with the younger ones of the opposite gender at their older age, but they don't understand the feelings well and are psychically deranged.
Now, dmmh, do you recognise yourself in any of those?