People can either think "being a parent would be miserable" or "being a parent would be fun". The interesting question is what leads them to one conclusion or the other.
In general, my life is pretty great and full of loving friends and family. I think my children's lives will be great too.
Do you have a support team that shares the load with you? We don't have anyone we can rely on. My in-law that does fly out sometimes would rather play with their phone than be a grandparent and we can't trust them to so much as drive a dog to the vet because we're sure they'd get distracted or lost or just crash.
Kids are a ton of of work. Life is a ton of work. Combine those two things and have no "village" to speak of and it's often too much. There's no pop psychology needed to point out that for many people, kids aren't their best life. Kids aren't always rewarding. Sometimes the right answer is the easy answer: if you don't want kids, having one will not be a net positive for your life.
However, like I said above, the interesting part of the question is the why. I think the lack of a village and support network is a very plausible answer. My friends that are the happiest with children also have a robust Village and social life which is supportive of raising children. This includes social activities that don't force adults to choose between parenting and personal enjoyment. This can be as simple dinner parties a couple times a week where one parent will watch The Children Play while everyone else takes a load off.
I think another similar plausible explanation is a cultural expectation that parents replace their lives with that of their children opposed to integrating the children into their own lives.
If parents like camping, travel, gardening, or reading, they should what they love with their kids.
Fuck watching the wiggles or elmo 8 hours a day. That's insanity and needless suffering.