about other assistants because their performances do not bother me enough to ask. But, this is a nomadic profession and the ambitious and the mediocre move around a lot. mediocre because they get fired/pushed out.
Sitake likes his DC despite the underperformance. Which can reflect a few things, he doesn't understand or value certain techniques to be taught, or his loyalty to his friend is more important. Or he doesn't feel someone else would come in and establish the "culture" he wants.