They are happy... Not for me, but for them.
Don't really like talking personal... but anyway... 
I read for them what they cannot see. They have an eye that speaks. I take care of them when they are tired. I help my mother with her works. Feed my father when his hands are dirty. I try to be there when they need to talk. I spend my nights taking care of them when they get ill... There are so many other things I do, but I don't think it necessary to mention it all here. That must have been enough...
If you still think they would have been happier without kids, better ask them, if ever you get the opportunity. I'll give you $ 100, from my own pockets, if they say yes.
Why such a narrow view? You just take into account what you have to do? What about what others will do for YOU? If am too busy, my husband or my kids can cook for me. If I have a presentation, I know I can rely on them to cheer me up, to encourage me, to be by my side. I'd have people to take care of me if I'm tired. I'll have someone to tell about how my hypothetical conference went. And I would need someone in case it didn't go right. I would have people to assist me, to talk and share things with. It's not only what I would do for them, it's also what they would do for me.
It's not when I'll be some 40-50 years old hag that I'll realize and regret what I lost, afraid of being too overloaded. Life is lived only once, live it FULL girl. 
Loneliness or celibacy is always empty, somehow, some kind... We all need someone, some time. Agree, friends will try to be there when you need them most-- but family will ALWAYS be there, whether you need them or not. You won't need reasons for companionship when family will be there.
Stephanie Coontz, director of research at the Council on Contemporary Families, reports that marriage has changed more in the last 30 years than it did in the last 300. What are the facts now? It used to be that women believed if they didn’t get married early, they might miss the boat. In the 1950’s, the average age of marriage for women was 20, with the most women marrying at age 18. There were very few first marriages after age 24. Coontz states, “This is a different world than the 1950s. The average age at first marriage for women is now almost 26. For women with a B.A. it is more than 27, and for women with master’s or professional degrees it is 30. And there is huge variation within each average, so that more women now marry for the first time in their 40s, 50s and even 60s than ever before in history.”
This is a true study. Why do you think women with B.A. marry after they are 27? They need focus to get their BA. Not husband, kids, meals, school, household, etc...Women who seek more, marry later, if at all. You think of career life as the old stero-type lead by our parents. The type forced on them by us, their kids. Work as you speak of is the type of get-money-for-food-clothes-school. NO, I talk of something bigger, something that can cause a change. Sure you want to be a doctor, go do your shifts come home after duty hours, no problem to add in a husband and kids! But you want to be a woman who adds a chapter in books of tomorrow? Then marital life is just a big clot right in the middle of your way.
You think I am against marriage in itself? no! for the prophet (PBUH) has said “Marry those who are loving and fertile, for I will be proud of your great numbers before the other nations.” Classed as saheeh by al-Albaani in Irwa’ al-Ghaleel. I am against of the belief that a man in this era would be content, for his full life with one woman. The whole "true love" comes only once thing. I am against marriage for a woman who strives for more in her life. And definitely against the different views that the society holds for a single man as opposed to a single woman. They see a single woman as being at a loss for not marrying, and a single man as one who had not found the right one yet! What are we, goods on a shelf? If we are not "bought" then it is our fault, because we are not intruging enough? Because a woman is not complete with a career life only, she needs a "man" to be there and kids. A thousand NO! A woman is more complete than a man would ever be without family life. Sure she wants a family life no problem. But it isn't necessary to have a fruitful life like you have pointed out. She IS living life FULLY even if she doen't get married!!
And please do not start the whole "you can be successful with kids too". I am not saying you can not. I am stressing on the fact that you can do it more focused, determined without them. If you are the type who would be content enough when she/he sees gratitude and love in your husband's/wife's eyes. then I would go for your argument. But , however, if you want to see that in more people, who would be grateful of your work, then work for it. Both will not concide as a woman. yes, maybe 97+% of most successful MEN are married. How many women??
A woman has more obligations when married than a man. Tell me the numbers of men who walked out on their families to women?
