Are parents skinnier, more beautiful, more energetic, and less jaded at 50? I think that's just aging in general, and would apply to both parents and not. Anyway, that line aside, I think people are just pretty different and find different paths to fulfillment through life, even if it's not always obvious from the outside.
Personal anecdote only: My life at nearly 40 is a tremendously better version of anything before it, even my 20s. No drama, way more peace of mind, little to no job stress, some disposal income, many hobbies both old and new, and most importantly... time. I'm not constantly focusing on the needs of kids, just what my partner and I feel like doing (both in the instant and in life in general).
I lost my job recently and feel great. Time to breathe and regather, without having to worry about making kids homeless or moving them outside of their school system. Don't wake up to crying babies and come home to shouting matches and needing to cook for 4. No homework help, just learning whatever we want to. Or taking the night to just relax and watch TV, try a new board game, go out somewhere new, take a new class, spend time in nature. It's pretty great. Every time we see or hear about someone else's kids, we feel absolutely sure in our choice -- not that we ever doubted it to begin with.
Far from feeling meaningless, I get to pursue work and hobbies that bring fulfillment, because they're well considered and taken for want, not need, and not dependent on the financial or time constraints of someone else.
It's only shittier if your stop growing. At this age I'm still contemplating a new degree, making new friends, evaluating new careers and cities (and frankly lifestyles), all with the financial resources and mental and emotional maturity I never had in my 20s. It's pretty great, and it was the exact outcome I hoped for. I knew I didn't want kids as early as my early teens, and got a vasectomy in by mid 20s. Both turned out to be terrific decisions.
Probably this sounds selfish. In truth I've always really valued community, and spend most of my adult life working in nonprofits and mission driven orgs, along with making a ton of friends all over the world. I love people, just prefer adults with their own interests rather than kids that I have to groom into something. The idea of living vicariously through them has no appeal to me (as someone subjected to the same from my own parents, in a nearly joyless childhood).
Another big reason I didn't want to create children (as opposed to adopting) is that there are way more than enough Americans already, each of whom consumes dramatically more resources than a child raided elsewhere. From a social and global standpoint it's not really sustainable, and probably quite likely apocalyptic, to keep having kids without really having a plan for their future in regards to climate and democracy, both of which are in rapid decline. That seems more selfish to me than not having them at all. Adoption seemed like an accept middle ground, but my partner didn't want kids (adopted or not) so I didn't pursue it.
Are there people who just settle into a routine after 30, never really changing much again? Sure, but that could happen with or without kids. That's up to you to prevent, as a parent or not. But I know more happy childfree couples than I do parents. Maybe it's a self selecting crowd. The happy couples probably don't want to spend time with kids, so we find each other. Likewise, the happy parents tend to fade from my life and have their own play dates and whatnot. To each their own...