My parents have more sex than I have EVER had sex during a relationship lol not sure why it happens with other couples but I'd guess all of what you mentioned, plus stress. My parents keep their sex life going because despite being busy they set aside certain times each day, plus their work schedules allow for them to be together at night. With us kids, we never really got in the way, my sis never wanted the whole love and affection thing most kids do, and I was independent and didn't need a lot of attention as a baby. As for stress, they're so easy going, they don't let things get to them, really.