I think research has shown that child free people might class themselves as happier, but that parents report more meaning and purpose in their lives. There is something magical about bringing up your children that nothing else comes close to. The sense of you actually mattering on this earth is amazing.
I don't actually know where that research comes from, but I very much doubt it is just based on individual subjective opinions. Probably, more likely, analysing some kind of self reported happiness scale?
I don't think childfree single women argue that we are happier than parents. We tend to say that we are happier for not being parents ourselves. I, personally, made the decision to not have children at a very young age and I've never wavered from that.
However, after many years of being divorced and unsatisfactory relationships since, I am, at the age of 48, starting to wish I had a loving partner. It's not the be all and end all of my life, but something that would make me happier.
I also think that surveys like this depend on life stage too.
Some of us don't think that bringing up children is magical. I have had many magical moments in my life, most of which have involved experiences like climbing mountains or, more recently, watching my beloved dog recover from a serious illness. That's where I get my magic. Parents get it from their children. Fair dos.
I've also never felt the need to matter to the world once I've died. I have no desire to leave a legacy. I'm happy just to be. Me and my amazing dog. And hopefully a lovely man to join us. That's my happiness.