You sound exactly like someone who doesn’t have kids, advising parents and speaking with authority that you don’t have.
All of us have (had) parents, the majority of us have been raised by parents, and that includes the childless people. And even the people who weren't fortunate enough to be raised by one or two parents are entitled to talk about parenting, I'd say that they're even more entitled to talk about it.
Anyway, this talk of "it's so hard to be a parent!" mostly comes from middle-class well-to-do people for whom it shouldn't be actually hard to be parents, or at least not as hard as it objectively is to much poorer people.
I suspect they're mostly doing it (the complaining, that is) as a social signifier, the same way that part of the same well-to-do middle class people used to complain until not that long ago that they're over-worked because of all of the emails.