My experience renting a fair number of vehicles a year is that in most states, liability is included up to the minimum amounts required by state law. One exception is California, where state law allows the rental companies to not have any liability insurance.

In any case, the rental companies always try to upsell liability insurance. But any savvy renter will know that an annual umbrella liability insurance policy is WAY cheaper and covers WAY more than liability insurance offered by car rental companies.