That's an excellent point. When I was a baby, I always sat in my mother's lap in the front seat. Pretty hard to forget about me in that situation!
I hate to say it but it's entirely possible to innocently forget about your sleeping baby in the backseat. People are just absentminded. A while back, I forgot to close my sunroof and the interior of my car got drenched by a passing thunderstorm. Not on par with forgetting about a sleeping baby but a pretty absent-minded thing to do nonetheless. Another time, I drove away from a gas station with the pump still attached to my car because I went in to get a coffee and forgot to detach the pump before driving away. That was fun to deal with.
Years ago, a neighbor of mine was packing his car for a trip and had the baby and babyseat on top of the car. He thought his wife put the baby in the backseat and his wife thought he took care of it. So they backed out of the driveway and the baby and babyseat slid off the car as they headed down the street. 911 was called and the entire neighborhood emptied into the street.
Fortunately no serious injury in that case but the couple was never the same after that. It was a very shameful experience for them and within a year, the house was put on the market and the family moved elsewhere.
But it's also possible to maliciously murder a child this way and make it look like absentmindedness.