How do you know? People are notoriously bad at predicting what will make them happy/unhappy.

(fwiw, I'm not saying you're wrong)

Having kids is insanely stressful in the beginning. Your entire life shifts to revolve around them and its utterly exhausting. I could not manage having kids and maintaining friendships at the same time and lost most of my friends due to inaction on my part. As kids get older though for me ~10 years old, they begin to have a lot of autonomy. You see the results of all of your efforts and you realize that everything you have done has contributed to create these small awesome little people with their own thoughts and goals. I have not cried since I was a small child, even when close family died I just shrugged and trudged on. I am for the most part relatively unemotional. My kid came into my room the other day, just smiling and telling me about something and it was suddenly the proudest I have ever been, I almost cried in happiness. It was one of the most surreal things I have ever experienced. I am really looking forward to see what he does next in life and am actually happy about getting to find out.

For one thing, after your children have grown up and moved out, most people are really glad they've gone through parenting, have an adult child they are in relation with, potentially grandchildren to enrich their lives with etc.