Jiu-Jitsu Is Never Ending Learning

I have been asked by students “Once you are a black belt, is it necessary to continue to try to learn about Jiu-Jitsu?” The underlying assumption behind this question is: Don’t you know everything about Jiu-Jitsu by the time you graduate to your black belt?
I am currently renewing my Gracie Barra instructor’s certification course ICP which features short video interviews with many Gracie Barra professors from all over the globe. One of the common themes expressed by black belt professors with over 20 years experience in Jiu-Jitsu is how they feel a strong need to continue to study Jiu-Jitsu.
What can they possibly mean by this?