I am a Karate Yondan (4th Dan) who has been studying Karate for 32 years, and a Judo Shodan (1st Dan) who has been studying Judo, on and off, for 20 years, and I DON'T teach. I find it more important to study Karate and Judo for ME and to benefit ME.
Teaching is a choice, and it is a choice that not all Yudansha wish to make for various reasons.
I am willing to share knowledge, both in and out the Dojo, but I don't see that aspect as a duty or obligation.
I have taught in the past, and enjoyed every minute of it, and I was proud of my students achievments, but I found that it took too much time away from my own personal study of Martial Arts in general, and Karate in particular.