This feeling is interesting, and very new. During the Baby Boom, people married young (women at 22 or 23) and had kids young, not as a "capstone" to a developed life.
Do you have any insight into why you feel that way?
Not the OP you're replying to, but a parent nonetheless.
From basically the very moment you realize you're having a kid you essentially become the #2 person in your own life. Its more so for the mother, but even the father likely needs to do some maturing in preparation to being a good dad.
If you're hoping to be a good parent you need to start weighing every decision on a basis of where it leads to. There's simple things like affordability: deciding to make smarter financial decisions because you're going to be supporting another human for the next couple decades at least. There's more complex things like suddenly seeing your own health as a liability and needing to straighten yourself out if you want to set a good example. Things get way easier if your life has structure honestly. Having good habits about food prep, cleaning, chores makes things way easier and bad habits really make life more difficult than it needs to be.
You also need to adapt quickly. You realize that you don't define your schedule any longer. Whether you sleep at night is up to the kid. As they grow from baby to toddler you start to see how the fundamental things like routine have impacted your child and you can begin to make connections to how you did things 6 months ago impacts how things are going now. Self reflection is huge, knowing how your emotions, your reactions, your behavior is a template this person learns from humbles you.
I think that's the new value system that leads to having children being such a burden. There are sooo many expectations of parents that they spend all their time fulfilling those without questioning any of them.
I always knew my dad was a good dad but only once I had a kid did I really understand how much he had quietly and willing given up because he loved me. More and more each day I understand him better and continue gaining an appreciation for all that he has done for me. All of this thanks to my daughter making me / giving me the chance to see life from the other side.
So that sounds like: in the past there were well defined templates for how to live your life, and people followed them; but now you have to make up your own, continuously figuring things out for yourself, and that is harder to do and needs more maturity.
Do you have any insight into why you feel that way?