Ever since Tyler Cestia left his son Thomas in his truck on a hot morning in June two years ago, he has felt, he said, like a cork bobbing in the ocean.

It was June 14, 2021, and Mr. Cestia was preparing for a stressful audit at work when he forgot to drop off Thomas at the babysitter on the way to his office in New Iberia, La.

At lunchtime, he drove to a restaurant with the auditor and then back to his office.

That afternoon, it occurred to him that he didn’t remember seeing the babysitter that morning. He ran to his truck where he found Thomas in his car seat behind the driver’s seat. Thomas, who was 2 and a half years old, was pronounced dead at the scene.

“It was just a total utter shock,” said Mr. Cestia, 37, who lives in New Iberia with his wife, Pam, and their two other children. “It’s almost like a nightmare that’s not real. I’m living in a makeshift world that’s not real. And once you come down off that, it’s a daily grind.”

Mr. Cestia said he has coped with the extreme grief with help from his religious faith and therapy. He has also had the support of his wife.

“People think, ‘Oh, how does somebody do that?’” Pam Cestia said. “You don’t forget your cellphone. You don’t forget this. But he was hyper-focused on something else. He’s not a bad parent. He’s not a bad father.”

The aftermath in other cases has been more dire. Marriages have fallen apart. Caregivers have been prosecuted and faced prison time. In one case last year in Chesterfield, Va., a father who realized what he had done immediately went home and killed himself.

And still the deaths come. Just this week in Houston, a 3-month-old died after he was left unattended in a car, the police said. About 40 children a year die from heatstroke in cars, either because they are left in the vehicle or because they become trapped, according to the National Highway Traffic Safety Administration.

That averages out to a child dying in a hot car every 10 days in the United States. The deaths are more frequent in summer but happen in every month of the year.

Child-safety advocates said that new technology could help prevent these recurring tragedies. Vehicles with interior motion sensors, for example, can sound the horn and send alerts to a driver’s phone if they detect a child in the back seat after the car has been turned off.

But automakers and regulators have not made the technology standard equipment in new vehicles, frustrating safety experts. According to Kids and Car Safety, a nonprofit group, 1,050 children have died in hot cars nationwide since 1990 and at least another 7,300 have survived with varying injuries.

“It should really be embarrassing for the automakers and to the government that this has not already been taken care of,” said Janette E. Fennell, the founder and president of Kids and Car Safety. “When you have the technology to prevent these deaths, and it’s not expensive, what are we waiting for?”

Federal regulators said they were developing rules that would require new vehicles to have lights and chimes to remind drivers to check the back seat after they turn off a car, as required under the $1 trillion infrastructure law that President Biden signed in 2021. But that requirement won’t take effect until 2025.

Major automakers have also pledged that by 2025 all new vehicles will include basic back-seat reminder systems. As of last October, more than 150 models offered the reminders, according to the Alliance for Automotive Innovation, which said in a statement that the industry was making “major progress” toward its goal of universal back-seat reminders.

But higher-tech systems that use radar or ultrasonic sensors to detect a child in the back seat remain relatively rare.

“It’s a matter of cost and demand,” said Emily A. Thomas, the manager of auto safety at Consumer Reports, which holds the position that child-detection systems should be standard in new vehicles. “People don’t know this is what they need, so there’s not a huge demand for it and, unfortunately, the auto industry responds to what’s required. So if it’s not required, they won’t put it in as standard equipment.”

About half of all hot-car deaths lead to criminal charges ranging from child endangerment to murder, according to Kids and Car Safety. Many parents and caregivers take plea deals to avoid jail time and because they are unwilling to face a court battle after the death of a child, the group said.

The psychological underpinnings of the problem have been discussed for years, at least since 2009, when Gene Weingarten of The Washington Post wrote a Pulitzer Prize-winning article exploring whether criminal charges are really appropriate for parents who accidentally kill their children by leaving them in cars.

David M. Diamond, a neuroscientist at the University of South Florida who was quoted in that story, has been patiently explaining the issue ever since, including for a documentary film, “Fatal Distraction,” that was released in 2021.

Dr. Diamond said that many of the deaths happen when parents drive to work and go into “autopilot mode and lose awareness of the child in the back seat.”

During the drive, the part of the brain that handles habitual behaviors like commuting “outcompetes and suppresses” the conscious memory system, which is responsible for reminding a parent to stop and drop off the child at day care, Dr. Diamond said. Stressed and sleep-deprived parents are particularly susceptible to this problem, he said.

“That’s why we need technology because, frankly, we are so forgetful,” Dr. Diamond said in an interview. “I try to emphasize to people that it’s not negligence, it’s not bad parenting, it’s just part of being human.”

The reminder lights and chimes installed in many newer vehicles advise drivers to check the back seat when the car is turned off. Those systems are usually triggered by a rear door being opened before or during a trip, but they cannot actually detect whether a child is in the car.

Ultrasonic sensors, found in some Kia and Hyundai vehicles, can detect a child (or a pet) moving in the back seat after a vehicle has been locked and then blow the horn and send text messages to the driver. But ultrasonic sensors may not detect a child sleeping in a rear-facing car seat, Dr. Thomas said.

Radar-based systems can purportedly detect even slight movements like the rise and fall of the chest of a child sleeping in a car seat. At least one vehicle, the Genesis GV70, features that technology.

In March, the Federal Communications Commission approved a specific frequency for short-range radar, which automakers say will make it much easier to deploy child-detecting radar inside cars. Before that, companies had to seek waivers from the F.C.C.

While radar technology is not widely available, safety advocates said that drivers could remind themselves to check the back seat by putting something important next to the child, like a purse, phone, wallet or even one of their shoes.

The Cestias have their own system. Every morning at 8:05 they text each other to make sure that their 1½-year-old was dropped off at the babysitter.

They have also spoken out strongly in favor of mandatory child-detection technology in cars.

“This is my opportunity to be Thomas’s mom and to advocate for him,” Pam Cestia said. “His story can help save other people’s lives.”