This is a crazy assumption and really insulting to parents or non-parents and wrong on nearly every count.
Don't know why I'm being downvoted. Almost everything assumed in the GP comment about me is wrong. I'm north of 40 and I have dependents. Don't blame your decision to have kids for your failures or deficits. It's a really annoying thing people do and inevitably gets projected onto the children, who didn't ask to be here in the first place.
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.
You do have a good angle that it is entirely possible. I dont think we're "blaming" children though, just the reality that a young single person with 10 years experience likely has a lot more spare time and energy than those older. Saying its easy to keep working long hours and learning new waves of technology likely aren't sustainable.