My oncologist uses a nurse practitioner

I think the main point to be considered is the level of training and expertise of a medical provider for a specific situation.
An MD is a medical doctor who can go on to become a Specialist in a specific area. They do additional training and are board certified.
PAs have generalized course work and do not have coursework for specialized areas of medicine during their schooling. They can chose to work in a specific area and acquire on the job experience. NPs can be more specialized, but they are far from a Board-certified Specialist
Expertise matters. For example, last year my cancer pulmonologist referred me for a consult with interstitial disease pulmonologist regarding best choice in fibrosis drugs. The desk set me up with NP and I said no, I need MD because I have both lung cancer and pulmonary fibrosis. The new MD Specialist reviewed all my tests, scans, stress tests for last 3 years. He then gave me a diagnosis saying I had at least 3 reasons for shortness of breath. And he decided I should go on Oxygen and hold back on fibrosis drug. No NP/PA could do that.
It is a logical choice. If one is a "vanilla" follow-up case, that is one thing, but people with complex conditions where treatment choices need to be strategic, one cannot expect a person of significantly less training/experience to provide sufficient level of care.