In those cases the distribution of responsibility is not likely to change post divorce.
Although, over time, I have seen many previously absentee parents come around later and want to be more involved. Sometimes it is difficult personality wise for the parent to deal with young children and/or the parent is a young child themselves (at least emotionally!).
The saddest cases to me are when there is a father who wants to be involved and do his part to carry the parenting responsibilites, yet the court bias awards primary custody to mom, and when mom is not willing and/or able to be the responsible parent, but she wants the financial gain only, yet the court still awards primary custody to the mom.