system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When jump starting a car, why do instructions say attach the jumped cars negative cable to the frame?

Because that has never worked for me, in any car I've ever jumped.

Every instruction I read, it says attach the negative jumper cable for the car being jumped to unpainted metal on the frame. Every time I've done that, in every car I've ever jumped, nothing happens. It's as if nothing is connected at all. When I attach the negative cable to the negative terminal of the battery being jumped, it works. When negative is connected to negative, the car being jumped indicates it's receiving some power.

It bugs me that the common instructions for this procedure have never ever worked for me, and that what has worked is something folks say to specifically not do. 

Clearly I'm doing something wrong, but don't know what.
gpt
It's mostly for liability purposes.

By jumping directly to the positive and negative terminals, you increase the risk of igniting potentially flammable gasses coming from the battery via the spark that often happens during the connection. These gasses can occur in certain scenarios, though is much more rare in this day and age.

If they tell you to connect both the positive and negative, and you cause an explosion, they could potentially be held liable. 

If you find a CLEAN ground, you *can* successfully jump a car in the manner described, and it IS safer.   But it is often hard to find a clean ground inside an engine chassis on a vehicle that needs a jump.