Why is it that the average RN career length is only nine years?

I was an RN in the ER for 14 years before becoming an FNP, and a Paramedic/ER tech for 8 years before going to nursing school. I knew several RNs in that time who had more than 20 years under their belts.
If it is indeed true that the "average" length is 9 years (Ive never read that), my guess would be, burnout. Its a very demanding job, with limited reward, under horrible hospital administrations in most cases.

Not true! I worked 44 yrs in oncology and ALL the nurses were there for decades in hospital and cancer clnic.