I don’t think these mothers forget that they have a kid in the car. I think the kid is sleeping and the mothers tell themselves *I’ll only be in the store for a minute and I’ll get back to the car before he wakes up*.
Maybe the dads who leave the kid in the car and go into work all day actually forget they had the kid in the car, but not the mothers.
I read the articles when I can, and there are all types, from drug users to otherwise-fine people who forget. The typical rule for forgetters is a change in plans...maybe needs to drop the kid at daycare on the way to work and gets a phone call...or simply puts his mind on work. I still remember one time one of my kids was in the car - I looked to my back right to make a lane change and the kid startled me...I completely forgot he was in the car. That was the last time any of my kids ever rode in the back seat with either me or the wife. Another common scenario is that grandma has the kids...again, something she’s not used to, so she forgets. My cars back then didn’t have air bags and I kept them a long time, just because of that...but had they had air bags, I’d still use the front seat.
The sad thing is that kids got pushed into the back seats after they were being decapitated by early-generation air bags (hundreds). They kept it relatively quiet out of fear of losing political support for air bags...so news stories about it were pretty sparse, but some info got out. Since then, airbags have gotten safer, with proportional inflation etc. - but I would never drive a car with an early-generation air bag - they had one setting-full blast and typically did a good job even on adults (re-arranged my mother’s face, in fact...thankfully plastic surgery put it back together).
http://articles.latimes.com/1996-11-28/news/mn-3765_1_air-bag-concerns