Ahospice client has no living relatives except for a nephew who has been put through college by the client, his generous uncle. why would the majority of states have laws that affect the nephew having durable power of attorney for the client?

I believe the answer is: As an heir, the nephew might be biased in healthcare decisions.

Even though the biased behavior that displayed by the relative is made with good intention, it might not be the best medical decision for the patient.
For example, biased relative is much less likely to authorize high-risk medical procedure even if those high-risk procedure is the best option for the patient.