You spend years training, and you finally earn your black belt. One thing leads to another, and you find yourself running classes more and more often, or you start a club or school of your own. You're going to get plenty of training in now, right?
Got news for you... Getting your own training in is one of the greatest challenges of teaching. If you're teaching the class, who is going to look at what you're doing? If you're teaching the students various things... when do you advance your own knowledge?
Teachers have to spend a lot of time that students don't see preparing for class. This includes things like putting a lesson plan together, and making sure the facility and gear is available and ready... but it also has to include your own training. I practice something for at least a little bit, every day. I don't feel I practice enough! There are things I can work on while teaching, like if I'm sparring a student, I can practice evading just enough... but there's a limit to how much I can practice that way. And there's a limit to how much the students may push me all too often! So a teacher also has to find time to advance their own training by working with others who are able to review and correct them.