This story sums it up so well.
I Accidentally Left My Child In The Car
It isn't bad parents that forget their children in the car, but simply parents, humans. Parents are humans too. They can make major mistakes.
As for the years thing, prior to the 1990s, where did children normally sit when they were with someone in a car? In the front. We used to fight over who got to sit up front. Hell, I remember being about 4 years old, and sitting in the front seat with my great aunt at a light, playing with the lock on the door, no seatbelt, let alone a booster/car seat (which most states now mandate booster seats until 6 and/or 60 lbs or 8 and/or 80 lbs, and they must be in the back seat til they are 10/12, depending on state). The door popped open on me, and I fell out. Sure, I'd not likely have been left in the car by mistake (although I might have been left inside on purpose while my caregiver went to do something), but I also could have been killed by a car if that light hadn't have stayed red long enough to get me back in the car.