That's why I put my kids in the car seats, as they sit higher I can see them every time I glance back at my rear view mirror. OR I buckle the car seat in front. Why parents forgot because they just let the kids sit without car seats, kids tend to fall asleep very fast and since no car seat the kids sure lay down. Then some dumb parents just forgot about them because can't notice any sound or the sight of the car seat....this is just my opinion
Don't put your small kid/baby in the front even though it's fasten with a seat belt using a kid's car seat, cause it's more dangerous in case of accidents. In some advanced countries (for example Canada), it's illegal to put kids up to a certain age at the front.