I'm not suggesting that people lock themselves in a garage with a running engine. In a separate post I mentioned the systems necessary to prevent the risk, like using the backup camera to ensure the garage door is open.

But I checked for a list of the most common causes of CO poisoning. It doesn't appear that anyone has put them in order or has broken down percentages, but the consensus seems to be that gas powered furnaces and appliances are the leading causes. Many of the websites on the subject actually list cars last. Such as http://www.carbonmonoxidekills.com/are-you-at-risk/sources-of-indoor-air-pollution/ and https://www.cdc.gov/co/faqs.htm

So, it seems your heater, water heater, stove, dryer, and even fridgeay be more likely sources. Again I'm not suggesting that people give it a try - but clearly if you open the garage door it's not going to kill you. Unless you're rolling coal, in which case yes those people should give it a try.