I completely agree with you. But I can't say around me, people, movies, society ever sent the message that parenthood is terrible. The way I interpret the message was always "this was hard, this was grueling, but I wouldn't change it for the world". I didn't get this apparent contradiction until it was my turn.
I think the people who miss almost all of these "tickboxes" and can somehow link it to the kids become unhappy with parenthood. If they ever feel their personal time, romance, career, success suffered because of the focus on the children, they will end up in the "disappointment" camp.
I never personally met anyone who was willing to admit regret of having children but I did meet people who were disappointed and put a veiled blame for their failure on the children.