I don't think its that surprising when you consider that (as I have stated before in other threads) we only have so much time outside of our responsibilities (family, work) so many people want to spend that time training and not teaching.
As for being able to teach and train at the same time, I agree with Paul (paw) that it's not uncommon for an instructor to spend less and less time training (see my "Why teachers stop learning thread" http://www.aikiweb.com/forums/showth...&threadid=4436
). It's not for lack of desire (for the most part) though -- it's for lack of time. And while many people counter-argue that they "learn something" every time they teach (and I do not dispute that) the amount that they "learn" while teaching is no where near the amount they learn when their mind and intent are focused strictly on learning and all their time is dedicated to training. Maybe some people can simultaneously learn and teach, but I myself am not that gifted. I need to pour my entire soul and being into to what I do, whether that be learning OR teaching, in order to get any value out of either (but hey, that's just me).
And when you say it is sad for them to leave rather than confront their sensei…there are a couple of factors at play there. One, some sensei's clearly take advantage of having dans around to teach so they can have a life outside of Aikido -- sometimes that is good, sometimes that is bad (most people have encountered both kinds).
Two, most students of Aikido (and Budo in general) feel honor bound to their sensei and would never refuse sensei's request for them to teach (if asked) -- but are torn since they also have lives to lead and its darn hard to juggle relationships, work and Aikido training -- much less Aikido teaching (which requires you to make a regular time commitment without fail). Remember that as a student, when you are away the only person that suffers is you…but when you are the teacher you have yet one more responsibility in a world of responsibilities.
Don't get me wrong, teaching is an excellent tool to help you develop -- but if it goes beyond an occasional development tool (as Jun stated "irregularly" in his poll), then it becomes a responsibility…and not everyone wants that.
I am taking this off topic perhaps though, by and by though I think I will train-only for another 13 years and then think about teaching