When I look at my brother and his wife, I see how difficult it is to have small children and health problems at the same time. My husband loves his children, but they do make his health problems worse, and they have caused health problems for my sister-in-law. Not all parents have health problems, but in their case it really makes their health worse. For them, it's all worth it because they deeply love their children and are very committed to them. I couldn't do it, though.
I have health problems, too, and children and my health don't go together (especially my wrists and neck, and my ears - I really cannot stand noise, it hurts a lot). I can't even tolerate my sister-in-law's dog because he often barks so unexpectedly, and it's so loud. (I adore the dog, really, I love dogs. But if I ever get a dog it will have to be a quiet one.)
So yes, I'm thankful that I don't have to lift children, or look over my shoulder (impossible with my neck - but if you have kids you have to keep an eye on them constantly), or do other things that I can't do with my health problems.
Also, it's nice to have a lot of time for my husband.
And finally, my parents are disabled and my father-in-law lost his wife a month ago, and we have more time and energy to visit and take care of our parent(s) because we don't have children. My parents appreciate that, so I'm not the only one who's thankful.