My understanding was that car rental rates in the US always included the minimum legally required liability insurance for whichever state you are renting in. They will always try and “upsell” you on buying more insurance, but what comes included is what is required to drive on the road (and will cover, to a point, the other guys costs if you get into a crash). It won’t cover damage to you or your car, in the event of an accident or theft, but you won’t get in trouble for driving without insurance if you get pulled over.
Is that still the case? Was it ever? It seems now at most major rental companies (Hertz, Budget, etc) they imply you get NO insurance unless you pay for it on top of the rental rate. So, unless you opt for the insurance, you are only legal to drive if you have personal car insurance that covers you driving other vehicles (and carry proof of it with you). Is that actually the case now, or have they just got more nefarious about how they describe the insurance options.
Most on my experience has been in California and on the west coast FWIW.