It's because the child feels safe nowhere. Conflict is bad for children, and if you put them more in the middle, it's bad. Expenses are greater in maintaining two full residences. There is duplication in need for clothing, furniture, and other necessities. This at least partially relates to the long-term financial strain.
Fathers are sometimes unprepared for the actual responsibilities of shared custody.
Despite the influence of the feminist movement, the reality is that most women are still the primary caretakers of children, even when they work full-time. Some fathers grow easily into an expanded role, others have more difficulty. When parents have unresolved marital issues, joint custody can exacerbate family conflict. This fact is troubling because joint custody requires much more frequent discussion between partners, and this contact can compound the conflict and animosity already present. The new spouse may resent the constant presence of the ex-spouse in the fabric of every day life.