A distinction between hot car deaths and drowning is that with drowning, the parent only need be inattentive for minutes. Hot car deaths require hours and hours of distraction and neglect. That's probably why they get more press. As to why drownings happen more often, that is why. Because parents being distracted for 3-5 minutes is more common than parents being distracted for 5-10 hrs.
In this case, BTW, it was 9 hours he left those kids in the car. That's a long time to not realize what you've done.
It takes a few seconds of forgetfulness and depending on the temperature only 15 minutes to become fatal.
Not paying attention to a kid while swimming is intentional neglect.
The stories get traction because they are rare and they let people look down on others for being human. Meanwhile no stories about all the kids injured and killed because their parents couldn't be bothered to properly install a car seat or adjust it for the kid.
The actually happened to one of my mom's co-workers 25 years ago. He was an extremely nice guy and a good dad. It was absolutely devastating. The company had on-site day car in the parking lot. He picked up his daughter for a check up at his normal lunch time, afterwards he grabbed some drive through and she fell asleep in her rear facing car seat. He was thinking about an upcoming meeting he was late for. When he got back to the parking lot he just found a spot like he did everyday after lunch and ran inside for his meeting.
Easy to claim that he was dumb or horrible and that you'd never do it yourself, but telling yourself that you'd never do that makes it more likely you will.