I also had the same question.
This is the response I got when I googled it.

[color:"red"]For one, it's practically unheard of to find a credit card that includes liability coverage. Travel insurance and third-party rental car collision insurance products don't generally offer it, either. Even American Express's fancy Premium Car Rental Protection doesn't touch liability, only damage to the rental car.
A Quick Primer on Car Rental Insurance - Autoslash.com[/color]