It's because the experience of so many parents these days is exhaustion. There just aren't enough hours in the day. In my experience it often turns out that if a fellow parent has significantly more personal time than me (for side projects, exercise, whatever) it turns out there is an unequal distribution of labor in the household. For example, a spouse who doesn't work who handles all the domestic stuff could allow the other spouse more free time. That same stay-at-home-spouse could find themselves with a lot of free time as the kids get older and require less hands-on parenting. That sort of thing.