I would argue US bonds are used as the risk free rate for lack of a better metric.

In theory you could adjust US bonds for the level of default risk but that adjustment itself would be very subjective.

People have tried to come up with better metrics like "swap rates" but they all fall short IMO.

A risk free rate is not real but neither are the models that use risk free rates. These things are only us trying to apply logic to reality which is often not logical.