It has to do with their regional insurance policies.
One can rent fun cars like BMWs in Italy - they are covered there locally.

I see 2 options:
Both Sixt and Avis allow drives into Italy

If you use your credit card for insurance you have to comply with the rental agreement. Since Hertz insurance is not involved, Italy shouldn’t matter for them in this case.
I suggest you communicate with Hertz in Germany in advance about the possibility of endorsing your drive into Italy if you use your CC insurance. Inquiring at the time of rental might be too late as the local reps might not have the authority to do so.

Keep in mind, that if you use the basic CC insurance that covers up to $50K you can easily exceed that limit with a simple car like Audi 5 convertible or a boring E-class Mercedes due to ridiculous European taxes.
Always check the total loss price of the car visible before they hit the print button for the contract