Have you ever babysat a kid, or worse, multiple kids? It only takes a second of distraction for a toddler to disappear in a crowd.

And that commercial you hear on the radio sounds a lot less stupid when you realize that the parents of most babies are incredibly sleep deprived, and sleep deprivation often causes the brain to be prone to running on autopilot for common tasks. That makes it very easy for someone to forget about anything not part of the default routine, like the baby in the backseat when they'd normally be at daycare. It has little to do with carelessness or intent. Here is a good article about it

I realize that it's comforting to think that accidents only happen people who did something to deserve it, but thats not how the world works. Sometimes, bad things still happen even if you're trying to do everything right.