Because it’s much more difficult to have good sex, uncomplicated sex when there is a baby in the room, you might wake the baby, you haven’t been to sleep for a week, you haven’t showered in 48 hours, you are bleeding from the nethers, you are leaking from your breasts, you are healing your body, you are touched out, your insides feel like jelly, it stings when you pee, you might wake the baby, you might wake the toddler, you’ve just spent five hours getting the toddler to sleep, the kids are in the bed again, the oldest is having night terrors again, the baby is screaming again, the kids are going to be up at the crack of dawn, somebody is going to sneak into your bed at 3am for snuggles, the oldest has growing pains again, the youngest is teething again, the kids can hear, you might wake the kids, the teens never go to bed, the teens can hear you, you’re going to make them need therapy, you’re sad because you miss the kids, you feel old and knackered.
It’s not impossible but it’s rather more of a task.