Separatists live in those cities, they were born in those cities, their kids live in those cities, their wives live in those cities, their entire family history is from those cities. While all of the less able to fight citizens of East Ukraine have thus evacuated to Russia, the remaining have stayed behind to fight for their homeland.
So did the Taliban. Separatists can choose what their fate will be; do they really want to follow the example of the Taliban? Or would they rather pursue peace? Up to them. Hopefully they will choose the latter.