They do, just not universally. The infrastructure bill included a section requiring them, so every new car will in a few years, along with systems to detect if you're impaired or drunk and refuse to allow operation if you are.

https://www.consumerreports.org/car-safety/auto-industry-agr...

https://www.cnn.com/2021/08/12/us/hot-car-deaths-infrastruct...