The easiest explanation could be monkey see, monkey do. Being raised by single mothers get to believe that that's the way to live.
Absentee fathers could also account for 'daddy issues', which makes the girls opt for 'older' men, sometimes married, sometimes just impossible to pin down. So they settle for being a single mom, as they've seen their moms do it and feel they can as well.
Just my five cents take on it.