I am also a parent with 2 kids, and, like you, I never had this happen to me when they were small. But I am not arrogant enough to think that is entirely down to my own awesomeness.
Like it or not, some parents just get unlucky. Through no fault of their own, they make a genuine critical error that results in the death of a child. It happens with cars in driveways, it happens with swimming pools, it happens with pets etc etc. There are hundreds of ways in which a small child can die accidentally and while most of us are pretty vigilant about these things most of the time, there is always the possibility that, in spite of our best efforts and intentions, we can make a terrible mistake.
It's not about paying attention, it's about the fact that we are not infallible. We are susceptible to being distracted. And when you have a confluence of random things happening in just the wrong combination, tragedy can result. That is not negligence, it's not criminal, it's shit fucking luck. Be thankful it never happened to you, and pray it never happens to anyone you care about.