I used to be quite critical of divorced men who are the "Disney/fun dad" thinking they're just irresponsible and don't want to do the hard stuff. However, experience has taught me that it's more complicated than that:

What often happens is that if the dad tries to be strict and discipline the kids, the mom will respond by being the "permissive one" and try to contradict the dad's rules. Then the kid won't want to come over to dad's house as there are more rules there.