Another factor is that when you have children and regret it you can suffer a lot more than just "-1" of misery - I'd say the scale is more like -10 to +1 (centered around 0). Furthermore if you end up regretting it you will also inflict some part of that misery onto your children.
Kids can be hard, especially if you're not a natural, and especially when they're young, but-- it's impossible to describe the fulsome love I have for my kids. I'm not a sentimental person, and probably wouldn't have had them without my wife (which is a funny thing to say). Even when they're a pain and a disappointment, even with the ever present guilt, my kids give me some baseline purpose for free. And the wins are great--"all joy and no fun" as I heard once. The conversations as they become real people. It's impossible to describe and totally worth it.
I don't know how much research there is on this but anecdotally there are many emotionally absent and neglectful parents out there. They might be able to cope with the way their life is but that doesn't mean that they're good parents (or that they wouldn't do things differently if they got to start over again).
There's also a chance that your child is born with disabilities that demand intensive life-long support. I know that I couldn't cope with that, so even if my general outlook on having children changed some day I could never get past this, for my own good and out of responsibility for my hypothetical child.
I only offer my own experience, because I had the same fears as you. One datapoint, but parenting changed me in so many good ways, and the hardships fade from memory.