I personally know someone (really a friend of a friend) who forgot a dog in their car. This person is on the whole, 99% of the time a fairly functional individual who works and has two kids. The other 1% of the time she’s struggled with depression, and lack of motivation. Not totally sure what my point is here :) - I guess only to say that people that are in this circumstance are probably not monsters - they’re more likely people that maybe in other circumstances we might call “spacey”. Maybe like the type of person who locks their keys in their car, or who leaves a burner going on a stove. I also think the points made already on this thread are good ones - the mandatory car seats in the back facing the rear, the increase in single parenting - are all factors at work here.
In my world, forgetting a pet in a car is senseless though forgivable but allowing your own child to roast to death in your car is CRIMINAL. These jurisdictions that take a slap-on-the-wrist approach to these cases are part of the culture of death and on-going indifference to infanticide.