I think 95% (made up #) of mothers say, "I have never regretted having kids" say that because it's what a mom is supposed to say, regardless of whether it's true. Someone earlier cited 1/3 number in an anonymous survey, and I am not a bit surprised by that. It feels awful, and like a betrayal to consider that one might have been better off in some significant ways without kids once those kids are real people and not just hypotheticals. It's too much like wishy little Sally and Timmy away. So I don't blame them. There's a lot of societal pressure there, as well as some cultural norms at play. I think that for the moms (and maybe dads, to a lesser extent) who actually might regret it, they can't even admit it to themselves in most cases, much less to anyone else. So that fact that almost no one ever says that doesn't mean to me that it is quite as universal as it might seem.
I am nearly 40, don't have kids, and don't plan to change that. (I am married and have been for over a decade.) I get that they make some people happy, and that some people would feel utterly incomplete even if every other aspect of their lives were nearly perfect, had they no children.
I don't have kids (which I find to be a common assumption when I tell people I don't want my own), and I don't look down on those who choose differently (defensiveness is another common reaction when I state my position on the matter). I wish my choice was generally accorded the same respect, but it often isn't. But that's okay, too, in the end. They are the ones who come off as judgmental asses, and I go on being happy.