I don't regret it at all, but I can definitely see why people do. I mean when you take everything into account, having a child can be the most catastrophic thing you'll ever bring into your life. It can permanently damage your body, your mental health, your financial situation, it ties you down, removes opportunities, you can have a child that will require care for the rest of their lives, you can find yourself isolated, you can be sleep deprived for a significant period of time. If a job that couldn't be quit led to all of that we wouldn't question people who regretted taking that job. And yet people look askance at anyone who acknowledges they regret having had their child!
It's a taboo but I promise you, there are many people who regret it. Everyone I've ever met who regrets it is a fantastic parent doing their best, they're just brave enough to acknowledge that if they could go back in time knowing what they know now, they wouldn't have done it. It's a taboo to share, hence why some people naively think 'you never regret the child you have, just the one you don't have, so go for it'. Because it's not really spoken about. There's an entire subreddit dedicated to regretful parents if you want to read about the experiences of people.
My own mother regretted it, as she told me when I was a teenager. Her biggest piece of advice was 'never have kids, it's not worth it'. But she had got married and birthed kids by her early twenties, as was the way in those days, before she'd really had chance to learn who she was or what she wanted, by the time she realised it was too late. She was an incredible mother and I'm so, so thankful to have her, but she wasn't willing to lie and pretend it was all roses. We are so lucky these days that the social norm is to delay having kids until you've lived a bit of life, then you can choose to have them if you want them but nobody is forced to. It benefits everyone.
IMO, and this is speaking as someone who fucking LOVES being a parent and all that it entails, if there's any doubt in your mind, don't do it. Better to regret not having done it than regret having done it, as in the latter scenario well, it impacts a child too. There are many benefits to being childfree.
One thing that isn't discussed often is the benefit to having one child rather than going down the path many parents automatically set off down where they have one then seemingly automatically have a second not too much later. With one child (as I currently have) it feels like the best of all worlds. I get to have him, be a parent, raise a family, and also get to have plenty of sleep, time for my hobbies and friendships and career and marriage. With a second I think the balance would tip and we'd no longer genuinely 'enjoy' parenting, it would become drudgery and much much harder.