There is a natural instinct for all species to pass on their DNA. People included.

I don't have kids and never wanted them. There are benefits to kids though, especially when you are becoming a senior, including having someone to pass on your legacy, having visitors, people to help you when those medial tasks like mowing the lawn make you feel like you ran a marathon, etc.

Also, kids can keep you feeling young. I have a close niece, so get the joy without the responsibility.

Plus it is the social norm (yes, many people really look down on couples that chose not to follow this established norm).

And even without kids, never are you free to do what you want. There are responsibilities that are there regardless of if you have kids or not. Kids don't add that much more responsibility. They are an extra mouth to feed, but you needed to eat anyway, so just make a little more food. You still have mortgages/rent, still have to work, still have to clean, still need to pay other bills, etc. You just don't need to find a sitter when you want a night out with your friends or spouse.