Most of these cases happen because of working memory problems, not because the parent is self-obsessed and negligent. Things like having a change in a routine can lead to a parent forgetting their child was still in the car. It is a tragedy, this parent is most likely devastated because their loved one is dead due to their memory failing. Parents get sleep deprived, they need more support. We don't need to point fingers and mock them.

https://www.consumerreports.org/cars/car-safety/anyone-could-forget-kids-in-hot-car-forgotten-baby-syndrome-a3901940661/

Diamond says the problem involves two parts of a person’s working memory: prospective and semantic. Prospective memory helps us remember to do something in the future, while semantic memory allows drivers to make the trip from work to home on autopilot, where they arrive without remembering clear details of how they got there.

Prospective and semantic memories work together to help us make changes to our routines; these changes can include things such as “drop off the baby at day care” or “stop for groceries on the way home.” When our working memory fails, such as when we’re distracted or stressed, there can be catastrophic implications, Diamond says. He gave examples of situations where critical safety steps were overlooked, such as a surgeon leaving tools in a patient, a pilot not setting the wing flaps for landing, and caregivers forgetting that there’s a baby in the car.

I remember when the flood hit texas and all those kids and folks died I completely lost it for days.

I hope that you find something to hold fast. And I hope that the 3 year old finds kindness in passing that humans did not provide.